Steam is a company making games rated "m" or lower one game i know is counter strike{some of you MAY know it},i think it is not sold in stores any more so here is a preview:
Steam has valve which made the orange box which contains Portal, and half-life and almost every one of the games they make you buy the game on your computer, download and play. I'm pretty sure all of them have an online where you play with other people and some of them like half-life is online and has a campaign. not all of their games have campaigns
Steam can also let you play games on other computers as long as you have an account,a game downloaded on a computer, a Steam console on that other computer, and the patience and time for the download of whatever game you have downloaded on the original comupter.