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The short answer is it's sort of like treasure hunting, but the treasure isn't buried and instead of a paper "treasure map" you're armed with GPS coordinates that have been published, which you use your GPS to help you locate to find the treasure. The fun is in finding the cache, not so much in any real value of the treasure (people put kids' toys, keyrings, and other small items in caches.) They have logfiles in them so people can say, "I was here! I found it!"
Any specific questions I can clear up regarding your Guadalupe River trip? Most folks use GPS's to keep them from getting to far afield from established trails. It definitely takes more GPS skill, coupled with map and compass skills, to do true bushwhacking (hiking/backpacking where there are no published trails) and not get hopelessly turned around or stuck on a ridgeline with no easy way down (except for that long drop with the sudden stop at the end kind!)