In WoWmod, you earn XP and level your characters as you go. If you play alone with bots only, the bots tend to have an higher level, and you can earn good XP here. You get loads more XP from killing high-level opponents.
You can level from 0-80 with normal gameplay. But if you buy and use the Cataclysm expansion pack, you have the possibility of going to level 85, and access even more powerful level 85 epic gear!
In some occasions, you get an higher XP-ratio, by example in Death Match Event, the XP rate is 25% higher.