+rep

Cases can and will drop. You don't have to subscribe to the special map series to get them but they are only around for a limited time and will eventually be replaced by newer cases with different items in each.

Ok, someone said something about cases.

Okay, so it's like this:
At the end of every full match, there's a chance random items will drop.
Cases require a key that typically needs to be purchased in order to open. When you open a case with a key, you get a random weapon. It's basically the same with sticker capsules. Stickers are something you put on a weapon, if you're into that sort of thing.
You can always buy and sell the items on the market much like the Team Fortress drops.
To use the items in-game, you need to swap them out in your inventory after you boot up Counterstrike. You should see a tab for that.