Posters have quite rightly detailed the procedures. I would only add a couple of additional points:

1) Even if you haven't applied for a visa before it isn't unknown to get a multi-entry if you apply. They will often say "no" when you go in person but if you explain to them nicely then sometimes they will relent. As mentioned, if you need to go to HK this is particularly useful as a quick trip there from mainland counts as an "exit" and a "re-entry"

2) If you're travelling on business then you do need a business visa but for that you need an invitation. If this is a problem, just go on a tourist visa. Last year the rules were very strict and I went in and out of China several times on a tourist multi entry visa

Loads of threads on this topic on the China forum

nickyboy