Just going to elaborate more on the drop system.
Each week, (Thursday-Thursday, may be earlier depending on where you live) each player gets an 11 hour window in which they get a chance to get new items dropped after they die in game. Like EP said, it can be a new weapon, hat, or tool (or crates) within the game. It is random what you get, and sometimes you get very lucky. Your drop chances fluctuate depending on your average playtime, but the drops only occur within that time window. If you didn't use up the 11 hours within that week, any time left will be added to your next week's time. This way, you don't have to play at least 11 hours a week. The rollover will continue to accumulate if you take a break from the game. Valve makes sure that everyone that plays gets the same amount of drop time no matter how often they play the game.
Some people capitalize on this by going to "Idle Servers", where their character will continuously die over and over. Since you only get your new items after you die, the repeated deaths increase your drop chances during the time window.
Personally, I just play the game normally. Any drop, whatever and whenever it is, is appreciated. Now, if only I could obtain that elusive Crate #40... :V