Not long ago, there was only the first Guitar Hero, in which you played a plastic guitar with five buttons, while the TV screen told you which buttons to press. Then came Rock Band and Rock Band 2, in which you and your friends could play together - guitar, drums and vocals all at the same time.
Guitar Hero World Tour is trying to catch up to Rock Band 2 by offering a full-band set-up of wireless plastic guitar, microphone and drums. You can play solo or with friends, or compete online.
If you've never played one of these full-band games but want to, you should probably go with Rock Band 2 by a hair. It's smoother, since it's been doing the full-band thing longer. But if you find Rock Band 2 too easy, check out the harder-to-master songs of Guitar Hero World Tour.
The trouble is you have to pay to download one new song at a time ($1.50 to $2 each). The other option is buying extra disks featuring about 20 songs for $40, like Rock Band Track Pack Vol. 2, but then you're getting songs you might not want.