When you find the treasure (cache), which is typically a small plastic container, you can log your visit to the logbook you find inside the treasure. In it, the treasure hunter writes that they have found it on geocaching, when and how many took part in the treasure hunt. In some caches there is a small thing, typically small plastic things, all of which have been lying in the drawer at home. Remember, if you take something from the treasure, so leave something of equal or greater value to the next. You then repack the cache and put it back where you found it.
There are other forms of geocaching and caches as well. Multicaches include several stops on a geocaching tour before finding the final cache if you want to be challenged more. If geocaching is completely new to you, you can learn more by reading the guide. Guide to Geocaching