A torrent indexer is a website

Website

A website, also written as Web site, web site, or simply site, is a collection of related web pages containing images, videos or other digital assets. A website is hosted on at least one web server, accessible via a network such as the Internet or a private local area network through an Internet...

, which hosts

Web hosting service

A web hosting service is a type of Internet hosting service that allows individuals and organizations to make their own website accessible via the World Wide Web. Web hosts are companies that provide space on a server they own or lease for use by their clients as well as providing Internet...

 and indexes torrent file

Torrent file

A torrent file stores metadata used for BitTorrent. It is defined in the BitTorrent specification.Simply, a torrent is data about a target file, though it contains no information about the content of the file. The only data that the torrent holds is information about the location of different...

s. 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

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.