Screenshots are fast and easy. And they guarantee screen resolution so your client or colleague will see the size you want them to see.
Command-Shift-3: Take a screenshot of the screen, and save it as a file on the desktop
Command-Shift-4: then select an area: Take a screenshot of an area and save it as a file on the desktop
Command-Shift-4: then space, then click a window: Take a screenshot of a window and save it as a file on the desktop
PC: Windows XP
Press the "Print Screen" key by itself if you wish to take a screenshot of your entire desktop. If you only want the contents of the window that is currently in focus, hold down the "Alt" key as you press "Print Screen." Regardless of whether you press "Print Screen" alone or with the "Alt" key, the contents of the screenshot will be stored on the Windows XP Clipboard. You may wish to edit with MS Paint.
PC: Windows Vista, 7
Same rules as above apply, but you also have access to the "Snipping Tool". Open the Snipping Tool by clicking the Start button, choose All Programs, choose Accessories, and then choose Snipping Tool.
Several of our users have recommended using Screen Print & Capture 32. It's free to use and can be downloaded here.