Screenshots

Windows System
On Windows systems, hit your Print Screen key. This copies the screen and places it in your clipboard. Now open any image editor and click Edit - Paste. Save this new image in any format you want.
In order to capture only the active window (the window that has the focus), hit ALT + Print Screen.
You can use any image editor, Adobe PhotoShop, Paint Shop Pro, FireWorks or even Paint.

Macintosh System
On a Mac, hit Apple + Shift + 3.

Linux System
Linux systems usually come with a free screen capture utility.