Here's how it works. Every two days Steam will post a set of objectives, which are generally game-related achievements. For instance, today's objectives require you to become a Desert Fox in R.U.S.E., unlock the Iron Curtain in Poker Night at the Inventory, and set a certain score in Chime. There's also one non game-related objective that requires you to upload an avatar for your Steam profile.
For a shot at winning 100 games, you have to complete any 10 objectives by the end of the promotion on December 20. You also have to log onto Steam on the 20 to be counted in the drawing. To keep things interesting along the way, Steam will give away five games to 20 people every other day for completing only one task -- and again, some of them are as simple as selecting a profile picture.
You can increase your odds at winning five games by finishing more tasks. Every objective completed gains you another entry in the drawing. Meanwhile, Team Fortress 2 players have a chance to unlock as many as three exclusive hats. You will unlock the "Bounty hat" for accomplishing any five tasks, the "Treasure hat" for doing 15, and the "Hat of Undeniable Wealth and Respect" for fulfilling 28 objectives.
It's also worth mentioning that Steam appears to be discounting any game required to complete an objective. For instance, R.U.S.E. is currently priced at $33.49 (33% off), Poker Night at the Inventory is $2.99 (40% off), and Chime is only $1.25 (75% off). We can only expect that more popular titles will be included as the contest unfolds, so be sure to keep an eye on things even if you're not participating.