Most graphical interfaces for Linux distributions allow you to take screenshots. These key combinations are the default on the most common distributions, and are usually remappable.
Press the Print Screen key on your keyboard.
Your screenshot can be found in the
Extra: Key Modifiers
There are other key combinations for taking other types of screenshot:
Alt + Print Screen: Take a screenshot of the current window.
Shift + Print Screen: Select an area to capture.
Control + Another Combination: Copy the image data to the clipboard.