What are TF2 keys?

In the computer game Team Fortress 2 by Valve Software, TF2 keys are used to unlock crates, which generally contain items with which the player can customize his character, such as hats. "Key" may also refer to the registration number needed to activate physical copies of the game on Steam.

Each type of Mann Co. Key opens a specific type of crate and disappears from the player's inventory after one use. Special keys are sometimes made available for the duration of limited promotions, and when the promotions end, these keys transform into regular key types, as the relevant boxes are no longer available.