Screenshot shortcut

This week I found a new way to make a simple screenshot, all of you know the PrtScr key in the keyboard, and how to use it (just paste it to a paint program). The problem with the PrtScr that it prints the entire screen, but what we can do if we have many windows open and we need the screenshot of one specify window?

The native way to do it is simply edit it in the paint programs, but for this purpose we have a 

Alt + PrtScr shortkey.

I use it about a week and its definitly one of the most useable shorstcuts.