OS X has several screen capture facilities built in. 

  1. command-shift-3 captures the entire desktop 
  2. command-shift-4 allows you to select a portion of the screen to capture
  3. command-shift-4 then space allows you to capture a specific window (move the camera icon to the window you want and left click)

adding control to the above modes causes the capture to save to the clipboard instead of the default location (desktop as a PNG file)