MacOS X features a built-in "screen grab" function you can use to take a snapshot of the entire desktop, a portion of the desktop, or an active window.
To grab the entire desktop and save it to a .PNG file, just press [command]-[shift]-.
To grab a specific portion of the desktop as a .PNG file, press [command]-[shift]-. When you do this, the mouse cursor will change to a set of cross hairs. Move it to the top right corner of the region you want to grab, click and drag the pointer to the bottom right corner, then release.
To grab a specific window as a .PNG file, press [command]-[shift]-, then press the [spacebar]. When you do this, the mouse cursor will change to a camera. Move it over the window you want to grab, then click.
Hint: If you add the [control] key to any of the combinations above, MacOS will copy the image to the clipboard so you can paste it into an application.