A torrent indexer (or indexer) is a website, which hosts and indexes torrent files. It usually lists files and shows their descriptions along with other information. These files can be searched for by visitors of torrent indexer, like their descriptions. Some of the torrent indexing websites have also tracking abilities.
Examples of torrent indexers that are used are Vertor.com, the Torrent Reactor Net, and Full DLS.com. Some indexer try to index as many torrents as they can, like Bitsnoop, while other focus on more specific categories, like TorMovies.