The word screenshot comes from two English words: screen and shot. It is a capture of the screen exactly as it currently looks.
This means the computer screen is recorded exactly as the windows, open programs and everything else are laid out on it.
Use
Screenshots are mostly used when you have a problem and do not know how to solve it. Instead of laboriously describing what popped up and where, it is better to take a screenshot and send it to whoever will help you.
A screenshot is most often taken with the Print Screen key. On a desktop keyboard it is usually above the Insert or Page Up key. On laptops it varies; it is often on the Insert key and used together with the Fn key. When you press the key, a screenshot is taken and placed on the clipboard, as if you had copied, say, some text. You then need to paste it into an editor, such as Paint.