[Avid]

Capture the whole screen:
Command+Shift+3

Create "cross-hairs" that allow you to select/crop just the area that you want:
Command+Shift+4

Capture just the open window where you click your mouse:
Command+Shift+Caps-Lock+4
(useful for capturing a plug-in window or error message)

These are all in the Apple Help Center, search for Keyboard Shortcuts, titled "Taking pictures of your screen".

All of these place the graphics file on your root level of your startup drive, named "Picture 1", etc.

This graphics file opens in Simpletext. If you want to edit it in a graphics program, or on a PC, you can rename it adding the extension .pct since it is actually a PICT format graphics file.