It's a good question, pretty much everyone has a different opinion. I used to think I was just really shy. Then for a while I thought I might have a mild social phobia, but now I'm back to thinking I'm just shy. From what I've seen posted on this website, most people feel that whenever you're in a social situation you get physical symptoms if you have a social phobia (like a racing heart beat, sweaty hands, unable to make eye contact, etc). I think that if you are just shy you may not be very talkative, but you don't really have any physical symptoms.
I'm 25 years old. I think my problem has gotten worse with age, but it's probably a little better now than it was a few years ago. I know when I was really little I would always talk to people while I stared at the ground. That was when I was real little though and I stopped that habbit fairly quickly. If I'm with one of my closest friends or with my family, I don't think I'm shy at all. I'm a little quiet around people I kind of know, but my main problem is when I'm around a group of people I don't know. I don't really get any physical symptoms, I'm just a little mentaly uncomfortable. Basically I just get really self-conscious.