from http://www.btinternet.com/~a.c.walton/navy/smn-faq/smn-faq.htmlCorvette: Small, generally slow escort-type vessel, generally intended for ASW. Best suited for coastal work but sometimes employed as a seagoing vessel. Often seem to be heavily armed for their size, but generally are lacking in things like sensors, electronics, reloads, range and accomodations.
similar to a corvette
Sloop: A multi-mission convoy escort type most common after WWI and prior to WWII. Slow, but with long range to operate with convoys. This type was mostly replaced by destroyer escorts/frigates during WWII.
Monitor: Monitors are slow vessels with minimal freeboard and extremely limited seagoing ability, equipped with one or more guns in armored turret(s) and relatively little secondary armament, intended for defensive ship-to-ship combat in coastal waters, or offensive shore bombardment. Monitors differ from coast defense battleships in the means of accomplishing the defensive role: coast defense battleships would meet the enemy fleet some distance out to sea, while monitors would wait along the coastline for the enemy to come to them. The coastal combatant role was a feature of the second half of the 19th century; the shore bombardment role appeared during both World Wars.
3rd Class Cruiser, Sheathed Cruiser, Colonial Cruiser, Gunboat (PG) The next step down the cruiser size scale, these ships were constructed for duty in areas where a presence was required but there was no significant threat. Thus they were typically slower and lightly armed and armored. In wartime such ships would have been supplemented by larger cruisers. They often operated as leaders for destroyers or small patrol craft. This gunboat classification only applies to the larger breed of gunboats; some other gunboats were small fast attack craft. This entire classification was mostly extinct by the end of WWI
Hopefully that will answer any questions on these classes. The address at the top is a good site- the summaries on each class are short but very well done.