Update 08/17/2009: I have just posted a guide that is a bit more extensive on exif data. You may want to read the new article for more information.
Exif data is metadata stored in JPEG and TIFF image files by digital cameras. It allows you to view the settings used for taking the picture, including the camera model, aperture, focal length, exposure, flash used and a lot more.
Exif data as seen in Windows XP
A subset of Exif data is readable by the Windows XP operating system. Simply right-click on the file, go to Properties and then in the Summary tab, pick Advanced. As you can see from the picture, some information is missing, such as the flash used.
Under Mac OS X, you can view Exif data with the Finder. Simply do a Get Info on a file and expand the More Info section.
Alternatively, you can use a free program such as Exif Reader (WinXP only) for retrieving Exif information from a file to obtain information not displayed in the file properties of your operating system.