From Wikipedia:

Geotagging (also written as GeoTagging) is the process of adding geographical identification metadata to various media such as photographs, video, websites, SMS messages, or RSS feeds and is a form of geospatial metadata. These data usually consist of latitude and longitude coordinates, though they can also include altitude, bearing, distance, accuracy data, and place names. It is commonly used for photographs, giving geotagged photographs.

Pictures taken with many newer cell phones are automatically geotagged, and software such as iPhoto, Aperture or Picassa is able to utilize that information. iPhoto and Aperture have their ‘Places’ feature, and Picassa has ‘Album Maps’. In an interesting reversal of the the ‘trickle down’ model of product development, many lower end cameras are now being offered with geotagging abilities, but most higher end DSLR’s are not. If you want to shoot high quality photos, and then tag them with location data, you have come to the right place. Using GPX files exported from Tracks you can match timestamps on your images to trackpoints in the GPX file and know pretty meuch exactly where those images were captured. Desktop software is readilly available for every major operating system to make this a fairly simple process.

Here’s how to do it:

Before you go start your photo shoot, take a moment to synchronize the time on your camera to your phone. Most of the desktop tools are able to apply a time offset in case you forget to do this step, but why complicate things. Your phone is actually one of the most accurate timepieces you can purchase, so why not use it to make sure you camera is applying correct timestamps?

Once your ready to go off on a shoot, open Tracks on your phone and start recording a session. If you’ll be out all day, you may want to adjust the Tracks settings to save a bit on battery usage before starting your recording. If you’re only going to be shooting for a couple hours, the defaults are fine, but if you’ll be out all day you should probably set the accuracy to ‘Medium (10 meters)’ and consider setting the ‘Distance Filter’ to a higher value. Once you’ve started your recording, just go out and shoot some photos. Multiple shorter sessions can work as well.

When you’re ready to offload your images from your camera you can export the relevant GPX files from Tracks to your main computer. You’ll also want to get a desktop application to help with the tagging process. The following suggestions are all free, and are known to work with Tracks GPX files:

• GpsPhotoLinker (OSX)
• GeoSetter (PC)
• GpxPhoto (Linux)

There are actually many other options depending on what your photo workflow tool is. GPX import and geotagging support is built into Apple’s Aperture out of the box. Maperture is a plugin that provides a richer experience and more options, and is now free. I typically use Adobe’s Lightroom, and Jeffey Friedl’s excellent ‘GPS-Support’ Lightroom plugin is my usual choice for actually writing the location data to the files. Jeffrey distributes his plugins as ‘donation ware’, and in my experience they work quite well and I’ve been glad to send a donation for all of his plugins that I use on a regular basis. In addition to using GPX files, this plugin supports a variety of other geotagging options.

For an excellent overview of geotagging in general, and on the Mac in specific, this article on Bioneural.net is quite good.