Chaznsc check out the root and Rom guide at the top of this forum. It can answer allot of your questions as far as how-to's. But to answer your question about limitations. You are limited by NOT being rooted.
You will not lose the market, there really isn't a reason to unroot your phone and for a beginner like yourself I would most defenitley recommend the Liberty 3 Rom. It's a fast, stable, and feature packed.
Go check out the rooting and Rom guides and come back when you have further questions. We are here to help in any way we can to make this process as easy as possible.