ImageMagick
provides a simple screen or window
shooting facility. To capture the whole screen:
$ import -window root root.png
Or the capture a specific window chosen with a click of the mouse
button, and including the window manager's frame:
$ import -frame -pause 5 window.png
This latter command will capture the screen region occupied by the
window. This might thus include other windows that overlap with the
selected window, so the pause option is useful to allow you you bring
the window of interest to the front. A simple mechanism for obtaining
a window shot is to run the command from the Run Application window,
usually initiated with Alt-F2. Or else, run the display command and
click the Capture button to get windowshot.