Steam Workshop
The Steam Workshop is a page located in the Steam's "Community" section. It is used to share user-created game content. The first game to support Steam Workshop was Team Fortress 2, later expanded to other Valve games such as Counter-Strike 2 (formerly CS:GO), Portal 2, Left 4 Dead 2, and third-party games (like 
The Elder Scrolls V: Skyrim) by various companies uses the Steam Workshop to host & get mods and other content. (ex. quests, weapons, textures, sounds, campaign, and much more.)
How it works
Subscribing
When in a workshop that allows you to download content, it will have a green "+ Subscribe" button. Clicking on Subscribe will download the content, but some games may need to be restarted in order to download the content.
Over time, when a workshop content is updated, you will get the latest version of the content when you launch the game.
For example: if you subscribed to a "Custom Portal 2 map" every time you start Portal 2 (assuming that's what it's used for) it would be at the latest version, ready to play.
Rating
When at a page for a Steam workshop item, you can rate it: Thumbs up or down. Though, when in Portal 2, at the end of a test, you are asked for a rating for the Steam Workshop, without actually going to the page. You also have the choice to comment on an item on it's page.
Non-subscribable items
Games like Team Fortress 2 and Counter-Strike 2, outside maps, have the Steam Workshop to user's disposal, but not to use to download anything. In this case, they are items crated by users completely able to be used in the game to be suggested. For example, if someone made an item call "EPIC HAT" (assuming it's a hat you can wear) and a high amount of people liked it, it could be added to the game.
Construction
The Steam Workshop is now fully available for the games that supported it.