To grab just a one-window screenshot, press Command + Shift + 4, then when the cross hairs appear, press the space bar. The cross hairs will change to a picture of a camera that can be placed over top of the window to be captured and clicked. The resulting window-only screenshot will appear on your Desktop.
With your mouse cursor over top of the word you’d like to look up, press Control + Command + D. A dialog will then pop up with the word defined by the built-in Mac OS X dictionary. While holding down the keyboard combination, you can mouse around to different words in the document.