As others have said, SA and AvPD overlap. Many people have both, but usually one is more severe than the other. I have severe SA but to a lesser extent feel I have AvPD.
SA: anxiety around people and in social situations, fear of being judged, fear of talking or just not being able to find your voice when you try, can enjoy having friends but are afraid they may end up in social situations or conversations that trigger their anxiety even with people they want to be around.
Ex: may go out somewhere but keep a low profile and hope no one will talk to or notice you; put on ear phones/have cell phone out/not dressing conspicuously so that people will think you're busy or won't recognize you.
AvPD: avoiding social situations altogether at all costs and as often as possible, avoids being around crowds or lots of people, can have friends but may avoid returning their calls or opportunities to hang out, and experiencing intense anxiety in uncomfortable situations that were unavoidable.
Ex: avoid going out altogether because you don't want to chance experiencing a social situation, or may take a different route that's out of the way instead of taking the route that most people are on in order to avoid crowds.