Using PrtScr

https://en.wikipedia.org/wiki/Print_screen

Many people don't understand the PrtScr (Print Screen) button on the computer. It used to print the screen contents. Now it usually copies the screen to the Clipboard, where other copied stuff goes. Alt-PrtScr only captures the active window.
Then you can paste it in documents where you can past a picture.

The Wikipedia link above has a fuller discussion, with additional options.