The Cruiser is a slow, long-range allied naval unit. It can be thought of as "water-floating artillery". It is good at attacking bases. However, its shells are not very strong and are very inaccurate, and it becomes cost effective against bases only at prolonged sieges, where the damage pays off over time. It cannot be effectively used for "hit and run" tactics.
The cruiser is vulnerable against aircraft and other naval units, and should usually be guarded by destroyers.