Social anxiety is a discomfort or a fear when a person is in social interactions that involve a concern about being judged or evaluated by others. It is a highly subjective feeling that is very hard to explain to other people, but ultimately it causes the person to be unable to fully engage in social interactions and social events.
It is typically characterized by an intense fear of what others are thinking about them (specifically fear of embarrassment, criticism, or rejection), which results in the individual feeling insecure, and that they are not good enough for other people.
In brain chemistry terms though the answer is more complicated than that, numerous things can cause social anxiety, and everyone’s form of social anxiety is slightly different, often it is a mixture of both psychological (such as learned helplessness) and actual brain disorder – certain mental dysfunction can cause anxiety by the nature of making the person lose social calibration or sync with others thus causing anxiety (the anxiety of which is actually fear of being weird)
Social anxiety is much much more diverse and complicated than the mainstream media let’s on. Those debilitated by it will suffer many social consequences of being unable to “integrate” into larger society.
Anxiety and social anxiety often overlap to the point where I don’t really know if there is an actual difference, anxiety is broader and more general, but they are closely related. The only difference is social anxiety may be caused by social cognition difficulties such as autism, whilst anxiety may be purely caused by other factors.