How to Take a Screenshot

Microsoft Windows
For any Windows version:

1. Press  (Print Screen) key to capture a screenshot of the entire desktop and places it in the clipboard. To capture only the active window, press both   and keys. 
2. Press V to paste the screenshot into other application like Paint or GIMP or Microsoft Word or LibreOffice Writer. 

Another option for Windows Vista and Windows 7 user: Use Snipping Tool to capture screen shots . It a free utility included in Windows Vista and Windows 7

Ubuntu Linux
Press to take a screenshot and save

iOS
Pressing and holding the Home button and the Sleep/Wake button. The screen will flash and the picture will be stored in PNG format in the "Camera Roll" on the iPhone or in "Saved Photos" on the iPod touch. The screenshot feature is only available with iOS 2.0 and later.

Mac OS X
On Mac OS X, a user can take a screenshot of an entire screen by pressing ⌘ Cmd+⇧ Shift+3, or of a chosen area of the screen by ⌘ Cmd+⇧ Shift+4. This screenshot is saved to the user's desktop, with one PNG file per attached monitor.