Just a very, very rich Bank I think, the richest in Essos (I don't think there are any Banks in Westeros).
Taken from the Wiki:
When princes or kings default on their debts or are foolish enough not to honor their agreements with the Iron Bank, the Iron Bank supports new princes and kings to appear. These new princes and kings then honor the previous debt along with paying back the money the bank loaned them in claiming their new power, lest they suffer the same fate as their predecessors.
AFAIK, mercenaries aren't typically used in Westeros, so they usually just back them financially.
Cheers. Was curious of their specific way to make sure they got paid back. Looking at you, Cersei.
I think I read it was an attempt to try and avoid confusion with the multiple companies. There's like a hundred of them in the book and they didn't want the users to get confused as to which company Daario belonged to.
I'm still disappointed they didn't go with the purple beard and the gold mustache.