To take a snapshot of the screen or a particular window on display, use the system-wide settings.
Command-shift-3 for the whole screen.
Command-shift-4 for crosshair selection of the content on the screen*.
Command-shift-4, with a release of those keys and then followed by a spacebar press to be able to snap the window you are mousing over.
Those will deposit screen shots on your desktop. If you just want them on the clipboard for pasting into another app without saving them to the desktop, include the control key in the mix.
* From Leopard onwards, some useful new features were added to the crosshair selection method:
1. If you hold down the shift key while sizing the crosshairs, you can limit the direction of the selection to the horizontal or vertical depending on which way you drag first (left or right for horizontal, up or down for vertical).
2. If you hold down the option key, the selection will size from the centre rather than from the edges.
3. If you want to move the selection area once you have sized it, hold space and you can drag it about the screen without it resizing.