Repeaters are used for study of the propagation in the different frequency bands and for contact between radioamateurs. The repeaters often are a meeting point for radioamateurs in a certain area or with a common interest. Momentarily most repeaters work in FM but other forms of modulation are possible like Digital Voice or Data.
A complete list of all the repeaters and a possibility for visualisation in "Google Earth" can be found on this page
Links to other websites with information about repeaters
Forum of the repeatertechnicians
Different links about repeatertechnics