SucceedSocially said:Check out the Wikipedia on Social Phobia for some good info on this question. There's obviously quite a bit of overlap.
Overall I'd say shyness is about being inhibited and not knowing what to say, but it doesn't have that horrible feeling of anxiety that goes with it. You may get nervous/embarrassed, but the anxious feelings aren't dominating the situation and being the main problem. Also, a shy person won't be as nervous about doing things in advance, while someone with SA will probably be a wreck leading up to it.