(This article was intended to be written on 18th Sep 2016, but actually written on 8th April 2017)
In order to take a screenshot on Mac, the standard way is to the Grab application.
Keyboard users often get used to these.
⌘ + ⇧ + 3 - full screen which generated a file on Desktop
⌘ + ⇧ + 4 - capture custom dimensions using the mouse.
I recently discovered that screencapture command is part of the OS X. I happen to have the need to paste a screenshot in a chat (in the current world, input boxes or gmail compose allows you to paste an image inline without the need to have a file on disk)
screencapture -c -i
solves most of my needs to capture interactively with output into the clipboard.