OpenCaching is a free source of geocaches which can be downloaded to a mobile device (there are free apps for mobile devices). This site explains exactly what geocaching is all about, how it works, how learners can create geocaches or search for existing geocaches shared by others. The site details the etiquette of setting geocache challenges as well as providing guidelines for users who find geocaches, and links to the free downloadable apps for mobile devices.
Geocaching.com is a US site which provides a host of background information about geocaching, how to get started and how to create or find geocaches. There is a Geocaching 101 which provides answers to a series of frequently asked questions.
Ollie Bray has written about the use of geocaching by primary schools. This post sets out how geocaching can support various aspects of the curriculum, and also provides links to further resources for using geocaching in an educational setting.
Jen Deyenberg, in her Trails Optional blog, has written extensively about the use of geocaching in the primary classroom in particular. There are several blogposts in the geocaching category on this blog each either giving examples of how geocaching has been used to support specific curricular areas, or how to go about setting up geocaches. The helpful gudies as well as illustrations of what actually happened in the classroom makes these useful for primary teachers looking for inspiration.