Press the "print" key (called "PtSc" on English keyboards AFAIK), its to the right of the F12 key or over the numeric pad, depending on the keyboard. The screenshot files are found in the installation directory, they have the name screenshotX.jpg, with X being a number.
You can also configure the screenshot key within the game's menu (under the Options -> Configure Controls). If you really want to get lots of screenshots or take them in the midst of heavy action, configuring a different key can make it a lot easier to get a good screenie.