Social anxiety disorder = extreme degree of fear of socialization to the point where it affects your life
Avoidant personality disorder = running away from social situations to the point where it becomes habitual pattern and affects your life
Both are probably tied together in many cases, AvPd is a common result of having SAD for a long time. Basically AvPD is just the clinical definition of cowardice, I don't think i would even call it a real disorder.