- For a full list, see Category:Booster Pack.
Once you have gotten all of your card drops for a game you own, you have a chance of receiving a booster pack. Boosters contain 3 random cards from the set. The odds of receiving a booster pack increase based on your Steam level: +20% for every 10 levels. In addition, Valve has said that there is an 'anti-bad luck' measure in play: the longer you go without receiving a pack, the greater your odds of receiving one in future giveaways.
Booster packs are given out in batches: a number of Half-Life 2 booster packs will be given out at one time for eligible Half-Life 2 players, Left 4 Dead packs at a different time for eligible Left 4 Dead players, and so on. The timing of when packs are awarded is not known, but is based on the badges that are crafted. As cards are used up from players crafting badges, those cards are replenished by pack giveaways. Thus the total number of cards in the system will not decrease as badges are crafted.
Booster packs can also be purchased using Gems. The cost of a booster pack, in gems, is equal to 6000 divided by the number of cards in the chosen game's set, giving a possible price range of 400 to 1200 gems. The booster pack crafting page can be found here.