there are lots of types of online gaming. there's online gaming where you just buy a game, like... battlefield or something (1942, vietnam or bf2) and install it, and install gamespy, get yourself an account, and play online, or there are mmorpgs. where you download the game or buy the game, get an account, get a login, pay shitloads to play each month, lose your life to the game, and just shit like that.

there are free mmorpgs, and all are fun. i recommend Diablo 2 for the best gameplay. that is free to play online.

the fantasy/sci fi mmorpgs are usually pay for a subscription to play online. standard shootemups like halo and half life 2 and quake and bf etc aren't pay to play. and you can jump on whenever you want and not have to worry about having to play for hours and hours and losing your social life for it to be worthwhile so you have an ubercharacter.

of course with WOW you can pay people to level up your character. but thats kinda lame and pointless.