I'm assuming you're new to the game, so i'll give you a quick synopsis
1) Role-playing character: you start out at lvl 0, and work your way up to the max lvl (20) by playing through quests, missions, killing monsters, and the like. This is the "storyline" side of the game. It is recommended (not required) that you start here, as you can "unlock" skills for PvP every time you learn a new skill. What does this mean? See #2.
2) PvP only character: cannot participate in any quests/missions/monster-killing, but starts out at the max lvl (20) and any weapons/items/runes/skills that you have unlocked with a role-playing character will be instantly available to you. You may then participate in PvP matches against other human-controlled players such as yourself to earn Faction (which can be used to unlock more skills/upgrades/etc).
Someone probably has a link to a better description than i'm giving, but this is the general "gist" of it.
