Geotagging is an aspect of photography -- especially DSLR photography -- that has grown quite a bit in popularity in recent years.
Essentially, geotagging involves adding information to a photograph's file that includes the location of the photo, the time of the photo, and the date of the photo, among other items. Such items don't appear in the picture, but they're embedded with the file.
With the right equipment, you can have your camera automatically add this information to each photo file, which can be especially helpful during travel photography. Click the link to read tips to help you more efficiently use a GPS unit with your camera.