Assalam o Alaikum! Friends today am posting how to take screenshot on both Windows & Mac OS.
How to Take Screenshot in Windows.
There are couple of ways to capture a screen in Windows.
So,the easiest of them is what we will be going through here.

• The first is by using the default screen-capture key on every computer(i.e The PrtScn) on your keyboard.
• Locate it & let's start.
• To take the shot of everything currently displaying on the screen including all open windows,press PrtScn.
• This will be automatically placed in the Clipboard.
• To capture just the active window,press Alt+PrtScn.
• Now to save the shots,open Microsoft paint by navigating to Start » Programs » Accessories » Paint and paste it there.
• Next select File from the top menu and choose “Save as”
• Give it any name you want and choose a directory to save the image.
• The second method involves using the “SPINNING TOOL”.This tool is by default part of  winidws8/7  & Windows vista.
• It is capable of capturing a specified area of a page instead of the whole screen as with the previous method.
• You can choose to use an amoeba-like[irregular] shape,a rectangle or capture the whole screen.

    How To Use “SNIPPING TOOL”.

• Go to start » All Programs » Accessories » Snipping Tool.
• Select a snipping type by clicking on the arrow next to “New” button.
• Select the window or portion of the screen you want to capture using your mouse.
• To highlight part of the screenshot,click on the pen/highlighter button in the menu.
• Likewise,to capture a menu/drop-down menu,press ESC after you've launched the Snipping tool,then open the menu you want to capture & press Ctrl + PrtScn.

These are for windows

How To Take Screenshot on Mac OS X.
Mac OS has an in-built screen-grabbing function,which enables you to take shots by clicking on some key combinations.When these keys are pressed,a click sound will be heard,then the screen-shot will immediately be saved in .PNG format.

• To capture the whole screen,press Cmd + Shift + 3 all at once.
• To capture just a portion of the screen,press Cmd + Shift + 4.
• Just drag the crosshair that appears around the area to be captured.
• To capture a specific window,press Cmd + Shift + 4 and immediately after that,hit the Spacebar.
• The crosshair will then turn into a small camera,highlighting all menus that can be captured.