I think it's the difference between having fear of social situations and therefore to avoid them, or to prefer not to be that much in social situations, because they overwhelm you.

There's no sharp definition of both that clearly differentiates them, at the end it's about personal traits that can't be clearly defined like a math equation, but IMHO the key thing for me is, if fear is the determining force and not your free will.

Please don't ask now about the free will. ;)