It can be kind of confusing if you run across a random NPC pirate while cruising around, and not know which class it is, as to wether you should run or fight, so here is the info, as acquired from the Marine Authority building on Laenor.

The following options are available for a ship of class 1:
1) Boat
2) Boat
3) Boat
4) Boat
5) Boat
6) Boat
7) Boat
8) Boat

The following options are available for a ship of class 2:
1) Sloop
2) Ferry
3) Lugger
4) Cutter
5) Cog
6) Curraugh
7) Ketch
8) Brig

The following options are available for a ship of class 3:
1) Schooner
2) Clipper
3) Swiftship
4) Brigantine
5) Corvette
6) Knorr
7) Caravel
8) Balinger

The following options are available for a ship of class 4:
1) Frigate
2) Merchantman
3) Whaler
4) Carrack
5) Crayer
6) Buza
7) Barque
8) Nef

The following options are available for a ship of class 5:
1) Galleon
2) Galleass
3) Man-of-war
4) Privateer
5) Indiaman
6) Viking Longship
7) Bireme
8) Dromond

The following options are available for a ship of class 6:
1) War Galleon
2) Destroyer
3) Juggernaut
4) Corsair
5) Dreadnought
6) Viking Warship
7) Trireme
8) Drakkar