QwertyWEB

                             Sometimes you require to take screen shot of your desktop in case you like the wallpaper or seem to have a problem or capture all desktop junk or want to send information to a client. And for this simple 10 seconds job , there are many many professional softwares like snagit , picpick ,  greenshot etc which you need to pay for for getting many quality features .
Still you don't need a dedicated screen capture app for this job as it is damn easy to do it from your keyboard for any OS .

#1 For Windows Operating System

On Windows systems, Press Print Screen key.
This button is most probably on right corner of your keyboard , adjacent to scroll lock .
This takes a snapshot of your the desktop screen and places it in your clipboard.
Now open any image editor like Paint , or MS office and click Edit - Paste or press Ctrl + V.
Now you will be able to see ditto pic of desktop screen as if you have taken a photo from a camera :P .
Save this new image in any format you want .
 
Note : In order to capture the active window Press ALT + Print Screen.
Other image editors that can be used are : Adobe PhotoShop, Paint Shop Pro, FireWorks etc .

#2 For Macintosh System [ Apple ]

Hit Apple + Shift + 3.

#3 For Linux System

Linux systems always come with a free screen capture app . You just check the app in terminal directory and choose the time after which screenshot must be captured . Works for KDE and other desktops like Ubuntu , Kubuntu , Fedora etc