In Starfleet, a Captain-rank officer typically serves as the Captain of a Starship. Of course there are also lots of Captains in other positions, especially on planets and in Starbases, and perhaps one could characterize "non-ship-captaining captains" as well, but still.
My question is: What do Rear-Admirals typically do in Starfleet? What are their typical duties, as opposed to Vice-Admirals on one hand, and Commodores on the other? Or - is it the case that all admirals are basically the same in term of duties, and its just a matter of time served in the rank?