For this weekend Team Fortress 2 is free to download and play at Steam, following the WAR update to the game.
What you need to do:
- Download the Steam Client http://www.steampowered.com
- Go to the deal page and click the 'Pre-load' link, alternatively go to steam://install/440/
- Wait for Team Fortress 2 to download
- Wait for the game to be activated sometime on Thursday
- Play the game
This will be completely free all weekend, you should not put any card or payment details in anywhere, if you do, you've done it wrong.
This will be a full version of the game, with no limitations apart from the time limitation of this weekend only.
If you decide you want to buy the game, you can do after trying it.
If you buy the game from Steam, you don't get a disc or a case or manual, but you can download it again from anywhere that has a computer with Steam installed and an internet connection.
I've put the end date as 21/12, as these free weekends usually go into the Monday, but I couldn't find anywhere saying the exact start and end times, so it may be early in the morning Monday that it ends, and it may begin mid-day on Thursday (due to the time difference).