Taking screen grabs is a common enough task for Web designers. Whether you need an image for reference or for a portfolio, having the right tool for the job makes things a lot easier.
Windows has a very basic screen grabber build-in. Hit Print-Screen (F13) and it puts the contents of the window onto the clipboard ready to paste into a graphics editor. Hold down the Alt key and press Print-Screen to capture the front window only. Okay, so you have to do a bit of cropping afterwards, but it's no big deal.
Screen grabbing on Mac OS is better catered for. In addition to full screen and window grabs, if you hit Control-Shift-Command-4 (it's easier than it sounds), you get a crosshair with which you can select any part of the screen. When you let go of the mouse button, the section is on the clipboard to be pasted wherever you like.
Also bundled with Mac OSX is the Grab utility which allows a timed grab, so it takes the snap after a ten second time delay, so you have time to rearrange things on the screen if you want.
One of the most useful facilities for Web designers, the ability to screen grab an entire web page, even the bits that are off-screen (and without resorting to multiple grabs and stitching the whole thing together) is left to third party developers. The cost of such utilities is not high and the added convenience and time saving makes them a good addition to the Web designers toolkit.
All these screen grabbers offer the same basic facilities but each has its own additional features that you may or may not find useful.