That's rather broad. I'll try to restrict my comments to the modern U.S. Navy, as my familiarity with other militaries is limited, and civilian naming standards vary from owner to owner.

Ballistic missile submarines are named after states, nuclear balistic missile subs are named after famous politicians, and attack subs are named after cities.
Aircraft carriers are named after people of military significance, including presidents.
Standard cruisers are named after famous battles, nuclear cruisers after states.
Frigates and destroyers are named after famous naval personages.

I think that covers it. Of course, these are only the modern standards - the standards in WWI and WWII were quite different.