Steam is a digital distribution, digital rights management (DRM) system developed by Valve Corporation that is both multi-player and cross-platform.
One of the advantages of purchasing games digitally through the Steam Store are that purchases can be played on any computer that has the steam client installed. Software purchases are permanently tied to your Steam account not a specific computer.
I don’t know how many times, I’ve lost or damaged game disks or misplaced licensing documentation for games that I had previously purchased. With Steam those potential problems are no longer an issue. Just download and install the Steam Client, enter your account credentials and you have full access to your library of purchased games.