It's important to make sure you choose a style that will reflect your personality, interests and taste. Here are a list of some examples for references sake.
Portraits - High detailed pieces replicating portrait photographs.
Realism - High detailed pieces replicating photographs, usually of wildlife or people.
Tribal - Large scale dark (usually black) custom pieces. (not to be mistaken for celtic)
Celtic - Intricate knot work (not to be mistaken for tribal)
New Skool - A new bright take on old school designs.
Old School - Traditional designs.
Bio-Mechanical - Abstract organic machinery, inspired by HR Giger (www.hrgiger.com)
Heraldic - Family crests and patriotic themed designs.
Lettering - The name says it all!
Japanese - Traditional oriental designs, usually incorporating dragons and koi.
Grey Wash - Smooth shaded designs using multiple tones of grey and black.
Full Colour - Bright eye catching designs.