A screenshot is nothing but a captured image of your desktop or laptop screen, saved to your computer. You can use it for many things, like capturing an error message that you receive, which you can then mail to a support center to resolve a problem. You can also mail it to someone, to help them look at a web page, without sending a link. It's also useful in solving someone's computer issues, by sending screenshots of the process, that they need to follow.
Here's how you can take one. Click on the window or desktop you want to capture, then press ALT+PRINT SCREEN keys on your keyboard. You can locate the PRINT SCREEN key (Usually labeled as PrtScn) on the upper-right corner. If you need to take a snapshot of your entire desktop, you can just press PRINT SCREEN without holding the ALT key.
After taking a screenshot of the desktop, you need to click on Start and then Accessories. Here, locate Paint, and open the program. When you open the Paint window, you need to click on Edit tab. Locate the Paste tab and click on it, to find your image pasted in Paint.
Now that the image appears, you need to click the File tab and select Save As option.
You can rename and save the file in any folder location, if you wish to keep it.
Follow these instructions properly, and you will have your screenshot saved and ready to be used, as and when you need it.