How do protectorates work?

I have yet to use protectorates since in 1.1 it was bugged and I never got into the habit. So now that they are fixed how do they work? What do they do?
I know that you need to slaughter the enemy and leave them pratically destroyed then offer the protectorate, so I just would like to know what they do and how they work.
Thanks

There are a couple of threads on protecorates...

Search back in Colosseum over the last month. Maybe two months...

look in the sticky above