I think either in this thread or that last we had this same or similar discussion if you might want to look farther into it that. Just saying this because there might be more info then what I might add here.
There are several different versions, the one I know about is that before the nervous break down (ie prior to the show starting) he was the unit chief, after the break down Hotch was named unit chief.
Now this is where it gets wierd, apparently the other story goes that when he was reinstated it was as unit chief, which moved hotch out of that position but into one where he would have to keep an eye on him to make sure he could handle coming back to work, eventually Gideon steps down from UC this allowed Hotch to take back over the role of UC.
In ep 2x23 "No Way Out (2): The Evilution of Frank" Gideon says the following to Hotch.
Gideon: Hey, Hotch. You're the best unit chief I know.
Hotchner: Jason, I'm the only unit chief you know.
Before joining the FBI, Hotch worked as a prosecutor for the District Attorney's office. It was referenced on the show about him being a former attorney, so that means except as an agent or when it is warrented to help a case he doesn't actively practice law anymore. In 3x1 "Doubt"
Gideon: How long can you hold that lawyer off?
Hotchner: Are you kidding? I was a prosecutor; I can hold him off for days.
Hope this helps some.