Steam is a digital platform for Windows, Mac and Linux operating systems. It works by allowing users to purchase thousands of games that can be stored on one single library accessible from anywhere. Here's what you need:
The software: http://store.steampowered.com/about/
An account: https://store.steampowered.com//login/?redir=about%2F
Purchasing games: Paypal, Credit or Debit Card, Paysafecard and more.
And this is how it should look:
Once you purchase a game through the interface, you'll find it in library ready to be downloaded.