Pro firms are about making money, simple as that.
You are pretty much right about there being two types of firms. The type where they employ you, pay you a salary and (maybe) part of your winnings, don't require capital and eat your losses.
These are, of course, difficult to get into, for obvious reasons.
The other type is the dressed up brokerage, like Bright, Echo etc. I am not trying to be mean, but there are very few differences between these pro firms and retail brokerages. Pretty much anyone who meets their capital requirement can get in and start trading.
If you've read these boards at all, you'll know that Don Bright loves to wax lyrical about all the advantages of trading with a pro firm. However, his is hardly an objective point of view. I'll spell it out for you.
With a pro firm, you have access to far more capital than the 4:1 margin of retail firm. Most props should give you about 10:1, and depending on the way you trade, much, much more. As Don would put it, "don't trade your own money...come here and trade the firm's capital." Which is just such a load of BS. If you take on huge positions you are very likely to wipe out your equity, and once of you have done that, you won't be trading the "firm's capital" for very long at all, will you.
Pro firms also give you access to "bullets". These are really only necessary if you trade listed stocks, as getting an upbid even on a fast falling Nasdaq stock is pretty easy.
With a pro firm, you'll most likely be trading in an office (although most are offering remote access.) THis may or may not be a good thing, depending on the people you are trading with.
That's about it really. The biggest "advantage" is the huge leverage you have. If you trade listed, then bullets are a big advantage too. Beyond this, there's really no reason to trade at a pro firm.