Cmd + Shift + 3
Will capture the current visible screen(s) into file(s) onto the desktop in PNG format.
Cmd + Shift + 4
Will enable a cursor to select a section of the screen for capture into a file onto the desktop in PNG format.
Cmd + Shift + 4, space, click
Captures the entire active application window into a file onto the desktop in PNG format.
Cmd + Space, "Skitch"
or select the heart shaped icon on the menu bar if sketch is running.
Upload to skitch/evernote or export to jpg/png. Skitch is a free app from the OS X app store which adds excellent annotation to screen capturing. Go get it now!
There are many more OS X command keyboard shortcuts that can really help your productivity. Keep good notes and keep the notes handy!