I'm not quite sure what you mean by these terms. Could you explain further, because I get the idea you're American and what you're talking about isn't what the average Brit is thinking.
As far I can see, I theatre art is one of a number of types of activity that goes on in a theatre - dance, drama, mime, music etc. People go to college to study theatre arts and this is what they study.
A theatre convention? That's either a large forum type meeting where you get to meet and see people from all areas of theatre and maybe attend seminars on one or aspects. Either that or it's a traditional/customary thing - like "it's a theatre convention that audiences sit in rows", or that stage left is the audience's right etc etc.
If I've got this completely wrong, then please tell me.