IMHO, regardless of setting, you generally have three types of ship which roughly correspond to wet-navy roles as follows:
Battleships - dedicated combat vessels, whose sole mission in life is to destroy the other guy's battleships.
Cruisers - multi-role vessels, with enough weapons and defenses to stand up to anything except the other guy's battleships, but not so much of an investment of resources that they can't be risked on exploration, scouting, or diplomatic missions.
Escorts - primarily intended to guard non-combatants from pirates and unforseen dangers. Tend to square off against their opposite number in fleet actions and essentially cancel each other out. Can be indispensible against unconventional threats (torpedo boats, submarines, etc.).