Here is what Wikipedia (the great "end-all" expert of all things) says about body dysmorphic disorder (BDD). Ok, so I know it is long. Just read what you want. This isn't the entire write up, just what I thought was interesting.
Body dysmorphic disorder (BDD) (previously known as Dysmorphophobia and sometimes referred to as Body dysmorphia) is a psychiatric disorder in which the affected person is excessively concerned about and preoccupied by an imagined or minor defect in his or her physical features. The sufferer may complain of several specific features or a single feature, or a vague feature or general appearance, causing psychological distress that impairs occupational and/or social functioning, sometimes to the point of complete social isolation. It is estimated that between 1%-2% of the world's population meet all the diagnostic criteria for BDD.
Onset of symptoms generally occurs in adolescence or early adulthood, although cases of BDD onset in children and older adults is not unknown. BDD is often misunderstood to affect mostly women, however research shows that it affects men and women equally. With a completed-suicide rate more than double than that of major depression, and a suicidal ideation rate of around 80%, BDD is considered a major risk factor for suicide. A person with the disorder may be treated with psychotherapy, medication, or both. Research has shown cognitive behavioural therapy (CBT) and selective serotonin reuptake inhibitors (SSRIs) to be effective in treating BDD. BDD is a chronic illness and symptoms are likely to persist, or worsen, if left untreated.
The Diagnostic and Statistical Manual of Mental Disorders defines body dysmorphic disorder as a preoccupation with an imagined or minor defect in appearance which causes clinically significant distress or impairment in social, occupational, or other important areas of functioning. The individual's symptoms mustn't be better accounted for by another disorder, for example weight concern is usually more accurately attributed to an eating disorder.
The disorder generally is diagnosed in those who are extremely critical of their physique or self-image even though there may be no noticeable disfigurement or defect, or a minor defect which is not recognized by most people. Most people wish that they could change or improve some aspect of their physical appearance; but people suffering from BDD, generally of normal or even highly attractive appearance, believe that they are so unspeakably hideous that they are unable to interact with others or function normally for fear of ridicule and humiliation about their appearance. They tend to be very secretive and reluctant to seek help because they fear that others will think them vain or because they feel too embarrassed. It has also been suggested that fewer men seek help for the disorder than women.
Phillips & Menard (2006) found the completed-suicide rate in patients with BDD was 45 times higher than in the general United States population. This rate is more than double that of those with clinical depression and three times as high as that of those with bipolar disorder. Suicidal ideation is also found in around 80% of people with BDD. There has also been a suggested link between undiagnosed BDD and a higher than average suicide rate among people who have undergone cosmetic surgery.
There is a high degree of comorbidity with other psychiatric disorders, often resulting in misdiagnoses by clinicians. Research suggests that around 76% of people with BDD will experience major depressive disorder at some point in their life, significantly higher than the 10%-20% expected in the general population. Around 37% of people with BDD will also experience social phobia and around 32% experience obsessive-compulsive disorder.Eating disorders, such as Anorexia nervosa and Bulimia nervosa, are also sometimes found in people with BDD
Common symptoms of BDD include:
- Obsessive thoughts about perceived appearance defect.
- Obsessive and compulsive behaviors related to perceived appearance defect (see section below).
- Major depressive disorder symptoms.
- Delusional thoughts and beliefs related to perceived appearance defect.
- Social and family withdrawal, social phobia, loneliness and social isolation.
- Suicidal ideation.
- Anxiety; possible panic attacks.
- Chronic low self-esteem.
- Feeling self-conscious in social environments; thinking that others notice and mock their perceived defect.
- Strong feelings of shame.
- Avoidant personality: avoiding leaving the home, or only leaving the home at certain times, for example, at night.
- Dependant personality: dependence on others, such as a partner, friend or parents.
- Inability to work or an inability to focus at work due to preoccupation with appearance.
- Decreased academic performance (problems maintaining grades, problems with school/college attendance).
- Problems initiating and maintaining relationships (both intimate relationships and friendships).
- Alcohol and/or drug abuse (often an attempt to self-medicate).
Common compulsive behaviors associated with BDD include:
- Compulsive mirror checking, glancing in reflective doors, windows and other reflective surfaces.
- Alternatively, an inability to look at one's own reflection or photographs of oneself; often the removal of mirrors from the home.
- Attempting to camouflage imagined defect: for example, using cosmetics, wearing baggy clothing, maintaining specific body posture or wearing hats.
- Excessive grooming behaviors: skin-picking, combing hair, plucking eyebrows, shaving, etc.
- Compulsive skin-touching, especially to measure or feel the perceived defect.
- Reassurance-seeking from loved ones.
- Excessive dieting and exercise.
- Comparing appearance/body-parts with that of others, or obsessive viewing of favorite celebrities or models whom the person suffering from BDD wishes to resemble.
- Use of distraction techniques: an attempt to divert attention away from the person's perceived defect, e.g. wearing extravagant clothing or excessive jewelry.
- Compulsive information seeking: reading books, newspaper articles and websites which relates to the person's perceived defect, e.g. hair loss or dieting and exercise.
- Obsession with plastic surgery or dermatology procedures, with little satisfactory results for the patient.
- In extreme cases, patients have attempted to perform plastic surgery on themselves, including liposuction and various implants with disastrous results. Patients have even tried to remove undesired features with a knife or other such tool when the center of the concern is on a point, such as a mole or other such feature in the skin.
- Chemical imbalance in the brain: An insufficient level of serotonin, one of the brain's neurotransmitters sleep and memory function. This neurotransmitter travels from one nerve cell to the next via synapses. In order to send chemical messages, serotonin must bind to the receptor sites located on the neighboring nerve cell. It is hypothesized that BDD sufferers may have blocked or damaged receptor sites that prevent serotonin from functioning to its full potential.  This theory is supported by the fact that many BDD patients respond positively to selective serotonin reuptake inhibitors (SSRIs) - a class of antidepressant medications that allow for more serotonin to be readily available to other nerve cells.  There are cases, however, of patient's BDD symptoms worsening from SSRI use.  Imbalance of other neurotransmitters, such as Dopamine and Gamma-aminobutyric acid, have also been proposed as contributory factors in the development of BBD. involved in mood and pain, may contribute to body dysmorphic disorder. Although such an imbalance in the brain is unexplained, it may be hereditary.
- Genetic predisposition: It has been suggested that certain genes may make an individual more predisposed to developing BDD. This theory is supported by the fact that approximately 20% of people with BDD have at least one first-degree relative, such as a parent, child or sibling, who also has the disorder. It is not clear, however, whether this is genetic or due to environmental factors (i.e. learned traits rather than inherited genes). Twin studies suggest that the majority, if not all, psychiatric disorders are influenced, at least to some extent, by genetics and neurobiology, although no such studies have been conducted specifically for BDD.
- Brain regions: A further biological-based hypothesis for the development of BDD is possible abnormalities in certain brain regions. Magnetic resonance imaging (MRI)-based studies found that individuals with BDD may have abnormalities in brain regions, similar to those found in OCD. 
- Visual processing: While some believe that BDD is caused by an individual's distorted perception of their actual appearance, others have hypothesized that people with BDD actually have a problem processing visual information. This theory is supported by the fact individual's who are treated with SSRI's often report that their defect has gone - that they no longer see it. However, this may be due to a change in the individual's perception, rather than a change in the visual processing itself.
- Obsessive-compulsive disorder. BDD often occurs with OCD, where the patient uncontrollably practices ritual behaviors that may literally take over their life. A history of, or genetic predisposition to, OCD or another obsessive-compulsive spectrum disorder may make people more susceptible to BDD.
- Generalized anxiety disorder. Body dysmorphic disorder may co-exist with generalized anxiety disorder. This condition involves excessive worrying that disrupts the patient's daily life, often causing exaggerated or unrealistic anxiety about life and circumstances, such as a perceived flaw or defect in appearance, as in BDD.
- Teasing or criticism: It has been suggested that teasing or criticism regarding appearance could play a contributory role in the onset of BDD. While it's unlikely that teasing causes BDD, since the majority of individuals are teased at some point in their life, it may act as a trigger in individuals who are genetically predisposed. Around 60% of people with BDD report frequent or chronic childhood teasing.
- Parenting style: Similarly to teasing, parenting style may contribute to BBD onset, for example, parents of individuals who place excessive emphasis on aesthetic appearance (i.e. that aesthetic appearance is the most important thing in life) or no emphasis at all may act as a trigger in those genetically predisposed.
- Other life experiences: Many other life experiences may also act as triggers to BDD onset, for example, neglect, physical and/or sexual trauma, insecurity and rejection.
- Media: It has been theorized that media pressures may contribute to BBD onset, for example glamor models and the implied necessity of aesthetic beauty. BDD, however, occurs in all parts of the world, including isolated areas where access to media mediums is limited or non-existent. Media pressures are therefore an unlikely cause of BDD, however they could act as a trigger in those already genetically predisposed or could worsen existing BDD symptoms.
I think most people suffer from a minor form of BDD but just not to the clinical level. In fact, I tend to call it body image dysmorphia. Most people with weight or size related body image dysmorphia, look in the mirror & see themselves as larger than they truly are. I seem to have the opposite issue. I look in the mirror & think I really don't look that bad, but as soon as I'm away from that mirror & in a social situations, I instantly picture myself as this enormous elephant of a person, waddling about the room. I imagine everyone is staring at me wondering why such a person would come to this event & hoping I don't waddle their way, try to talk to them, or knock someone over. I really don't know if a psychologist would call this BDD but all the same, it is an unrealistic image of oneself, right? I found the part about the psychological causes especially interesting.
I thoroughly believe this is where my problems stem from. When I was a child I never thought there was anything wrong with me until I was in 2nd or 3rd grade. Then I started getting teased for my weight. That taunting instilled in me the thought that what I saw in the mirror must not be the truth.
I still battle with this everyday but I am slowly gaining a confidence that I have never had. I attribute this to the exercise I am doing. I feel stronger & more confident every day. That fist day of walking into the gym was one of the most frightening things I have ever done. Some days, as I am walking through the parking lot to those doors my little demons whisper in my ear those self-loathing thoughts but their voices are diminishing & when I leave that building I walk out with sweat running down my back but my head held high & my back a little straighter.