Case Knives Wiki
Weapon cases are items that are obtained through random timed drops or given as Operation Mission rewards. The cases are locked, and can only be opened with a Key. Keys can be purchased directly from the ingame store and the Steam Market, or can be obtained by trading with other players.
When opened, a case will drop a randomly selected item from its drop list. As the potential loot from a case vary in rarity, there is not an equal chance of dropping each item. All weapons obtained from cases are at least of the Mil-Spec or "Rare" grade, with the coveted being the rarest. Weapons dropped from cases also have a chance of coming with a device, which tracks the number of kills made with that weapon.
Keys are items used to open weapon cases and sticker capsules. They are usually bought straight from the CS:GO store, but can also be purchased from the Steam Market or acquired through trading.