PC:

1. Print the whole screen
Print Scrn Button on your keyboard

2. Print a particular application window
alt + the Print Screen Button on your keyboard

Paste your shots into your document by holding down ctrl + v!

Mac:

1. Print the whole screen
cmd + Shift + 3

2. Print a portion of you screen
cmd + Shift + 4,
then use your mouse cursor to select the portion you wish to capture

3. Print a particular application window
cmd + Shift + 4,
release all key and then press the space bar,
then use the camera to select the application window

Your picture files will be on the desktop!