In Counter-Strike: Global Offensive, you can collect guns. These guns can replace the default in-game guns. They do not change gameplay at all, and are purely cosmetic.
How can I get new guns?
In Counter-Strike: Global Offensive, you can collect guns. These guns can replace the default in-game guns. They do not change gameplay at all, and are purely cosmetic.
How can I get new guns?
This is the new "Arms Deal" update which went live yesterday (14/08/2013). Valve released a really good FAQ on it, which can be found here.
The bits of interest for your particular question, "how can I get new guns?":
Regarding the first point, these are obtained for play time over the course of a week. As your play time increases, the drop rate will slow down. This is reset when the week rolls over. Not sure the start day or time of a week though.
I'm not 100% sure where you will get the weapon case keys. The below quote suggests that you buy these:
This update will also actively support the CS:GO competitive community with the introduction of the eSports Weapon Case. A portion of the proceeds from sales of eSports Case Keys will go towards prize purses at competitive events.
Hope that helps.