Apple - How do I take a screenshot on my Mac?

cmd shift 3 takes a screenshot of the entire current Desktop (all attached monitors, as separate shots].

cmd shift 4 takes a screenshot of a dragged selection.

cmd shift 4 then space (you must press space separately) then click, to take a screenshot of a specific window.
Holding Opt ⌥ as you click will remove the drop-shadow.

Add control to the shortcuts above to place the screen shot on the clipboard instead of saving it to the desktop.

Also you may define a shortcut key for copying the selected area to clipboard as I did.

Here is how:

1. System Preferences
2. Select Keyboard Pane
3. Select Keyboard Shortcuts tab
4. Select Screen Shots from left menu
5. Double click to the shortcut option of "Copy picture of selected area to the clipboard" and define a key that rarely used (eg:F13)

And done! From now on, when press that key, the area selection pointer appears and after you select the region on the screen, it copies the picture to the clipboard and you may paste anywhere.

Hope it helps...