A video game where the player controls a character in a world inside the computer. The goal of most MMOs are to advance your character, build up his armor, weapons, etc. Some MMOs actually allow you to buy houses from players who are architects, weapons from weaponmakers, clothes from tailors, etc. Many MMOs cost on average $15/mo (USD) -- although Guild Wars is the only current free MMORPG.
In sacrife of my entire summer, I mastered the professions of pistoleer, ranger, chef, tailer, and architect in the cool new MMO!