Social Phobia/Anxiety is an anxiety disorder where the person thinks that others will judge them negatively ("they'll think I'm stupid" etc), and it is therefore at its worse when with other people.
The attention is mainly self-focussed and the person indulges in mind-reading, in that they think they know what others are thinking of them and they try to interpret every glance and expression to give a clue as to what the other person is thinking about them.
Avoidance of social situations is a direct result because they don't want to feel the uncomfortable physical sensations of anxiety. They deny themselves the opportunity to learn that maybe the situation could have been OK and they could have handled it.
Safety behaviours are developed to help them cope with situations that can't be axvoided, such as having someone with them, avoiding eye contact, holding or fiddling with something, trying to hide or having an escape plan (e.g. sit by door or in aisle, make an excuse to leave early). This all increases the self-focus.
CBT for Social Phobia
Cognitive Behavioural Therapy looks at the cycle of Sociap Phobia and teaches them to challenge the unhelpful thoughts and beliefs, learn to control the focus of attention, and change the behaviour to something more constructive.