Pictures taken of your screen or sections of it are commonly referred to as Screenshots or Screen grabs. Both Windows and Mac OS have built-in tools and 3rd party software available.
You can create pictures of all or parts of your screen (which will be saved to the desktop in pdf format) using these commands ...
Mac OSX also comes with a simple application called Grab for creating screenshots. Find it in the Applications - Utilities folder.
Snapz Prox is an excellent program for creating a range of screenshots in a variety of file formats. It can also create timed QuickTime movies of your on-screen actions. Find it at www.ambrosiasw.com