Steam is a digital video game distribution platform developed by Valve. Through it users can buy and download a large number of games from different developers, not only Valve, which are currently in its catalog. Apart from all that, it offers several other services that range from achievements or lists of friends to play to automatic and instant updates.
How does STEAM work?
The operation is quite simple. When a user buys a game through Steam, what they are buying is a subscription to that game, a personal and non-transferable subscription that can only be accessed by the player when they start their account. The player will at no time receive a physical copy of the game (although he may create a copy of the game on some physical media if he wishes after downloading it) but this will be permanently linked to his account, and the user of said account will be able to download and install it as many times as it deems appropriate, and on different computers without any restriction apart from the fact that you can only access it from one computer at a time by accessing your account.
Some games on Steam are free to download, for others you will have to pay, and some are very, very expensive. Fortunately there are ways to get all Steam games for free as you can see from gonooo.com/steam-code-generator/, there are tools that can help.