When you get home and download your photos onto your home computer, you also connect the GPS unit to your PC via the supplied USB lead. The installed software starts automatically and prompts you to download the GPS log file, and to select the folder containing the photos that you want to track. The software simply compares the time code from the GPS data log file with the time recorded by your camera in the EXIF data of the image files, and from that works out where the photo was taken.
Obviously for this to work properly the time and date on your camera have to be set accurately, and it's also worth noting that the GPS time code does not include Daylight Saving Time (or British Summer Time) so set it to the absolute local time. In the UK at this time of year, GMT is on hour behind BST, so you have to take that into account. If you're visiting a foreign country in a different international time zone it's also important to remember to set your camera clock to the local time before shooting.
It is simple and unobtrusive in use, either hanging from its karabiner or using the Velcro-strapped cover, both of which are included. It has only one control; the on/off button. I tried the gadget out on a quick photo-safari around my home town of Exeter. After switching the unit on it took about three or four minutes for it to locate a signal (indicated by a flashing LED on the side of the unit), but once it has done so it recorded my position accurately for the rest of the afternoon with no fuss at all.