Is there a section that explains what each job type thing actually does? It lists needed skills and the name and that's about it.
Not in the current Character Creation Guide, which lists skills you get from having that job. The new guide (in progress decribes the jobs, however. If you need a description for any particular job, ask me and I'll describe it (and add that description to the new CCG if need be).
How, exactly, does the roleplay on this board work? You mentioned something about using both posting and Yahoo! messenger but it looks like a lot of opening posts with no replies in them...
We usually get together on YIM (JPing) since it's much faster that way that individual posts. Individual posts (SPing) are perfectly fine, though. Say, for instance you character was in mid-JP and someone had to leave and so the JP was paused, you could SP your character's next action (such as going to another area, etc.) The frequency of JPs tends to create many large posts that are a whole thread rolled into one. For timeline purposes, the threads are locked when replying to them would no longer make sense, time-wise. Each thread is, however, generally a continuation of the last thread/post in a very large ongoing story.
The RPers for that particular session get together on YIM, then IM out the RP. One person cleans it up (typos etc) and posts it.
Tim described the usual procress pretty well. Again, using the boards directly is allowed, too.
NOTE! If you're going to make a non-Star Army character (assassin, merc), please note that the support for missions and plots is limited and you'll find yourself free-styling it most of the time. It's not easy and doesn't give you much chance to interact with other characters. Just so you know.