Social Phobia (or the colloquial "social anxiety disorder") involves avoidance of (or dislike of) social situations in which the individual perceives himself or herself to be subject of possible scrutiny. It is the possibility of scrutiny (i.e., negative appraisal from others) that provokes the anxiety reaction in the individual with a Social Phobia.
Social Phobia has an exceptional amount of overlap with Avoidant Personality Disorder. In fact, many experts in the field contend that they are, in essence, the same disorder (i.e., they could be plotted on, say, the same continuum of psychopathology), with the Avoidant Personality Disorder representing a more pervasive, generalized, and severe form of Social Phobia. Notably, those who are diagnosed with an Avoidant Personality Disorder are often diagnosed with a Social Phobia, and vice versa. My clinical approach to this topic goes kind of like this: if you tell me that you've always been phobic of social situations because you don't like the idea of people scrutinizing you or thinking of you negatively, and you tell me that you've been that way since your childhood, and you tell me that it's never really went away (i.e., it has persisted across your lifespan), then I'm likely to diagnose you with an Avoidant Personality Disorder; but if you tell me that you weren't like that as a child, and it more recently developed (i.e., during, say, college or your early adulthood), then I'm likely to go with the Social Phobia diagnosis. All in all, though, this tends to be very academic (i.e., "splitting hairs"), and the take home message should be that the two disorders are essentially the same and both involve being really anxious because you don't like the notion of others' scrutinizing you.
So, let's say your dislike or anxiousness with respect to being around others has nothing to do with notions that they may not like you. Okay, well, we can effectively rule out both the Social Phobia and Avoidant Personality Disorder diagnoses. Instead, two other disorders, Panic Disorder With Agoraphobia and Agoraphobia Without History of Panic Disorder, become applicable. You may notice that the common word among both those diagnoses is "Agoraphobia," which, literally translated from its Latin roots, means "fear of the marketplace," but, more clinically, means a fear of being placed in situations in which escape may be difficult or the individual fears experiencing panic-like symptomatology. Panic-like symptomatology is related to autonomic nervous system arousal and involves what you'd think would happen in the event of autonomic (or sympathetic) nervous system overload: sweating, trembling, a fear of impending doom, a feeling of being suffocated, hyperventilation, numbness in the extremities, and so forth. So, people that have agoraphobia get really worked up (i.e., they get "panicky") in certain situations that involve being around peoples or in particular places.
There's a continuum on which you can place panic-like symptomatology. On one end, there's very little panic-like symptomatology, or none. On the other end, there are full blown panic attacks, where the person absolutely freaks out, thinks they are dying or having a heart attack, and is completely off the charts with respect to autonomic nervous system arousal. If you have panic attacks and, as a result of your panic attacks, you avoid going out in public because you don't want to have a panic attack in, say, the grocery store with a bunch of people around, then you have a Panic Disorder With Agoraphobia. If, though, you don't have panic attacks, but still get worked up in certain situations (e.g., in elevators, on planes, in waiting rooms, in crowded places), then you have Agoraphobia Without History of Panic Disorder. (You may notice that they key difference is the presence of panic attacks - if you have full blown panic attacks, then you win the prize of a Panic Disorder diagnosis; if you don't, then you don't qualify, and instead get put in the Agoraphobia Without History of Panic Disorder line.) Remember, in order to be a true disorder, the symptomatology must cause either clinically significant distress or impairment. In other words, sometimes you can have some traits or aspects of a particular disorder, but fall below the threshold necessary to establish the actual diagnosis because your symptoms are not severe enough to cause significant distress or significant impairment.
Hope that helped. Let me know if it didn't or you'd like me to expatiate on something else.
