Jackett is an open source application that allows users to access a range of torrent sites from a single interface. It is designed to be a unified search engine for all torrent sites and trackers, providing a single, unified search for all torrents. Jackett works by creating a proxy server between the user and the torrent sites and trackers, allowing the user to access torrents from a single, easy to use interface, instead of having to search individual torrent sites. It also provides automated summaries and filtering options to make finding and downloading torrents much easier. Jackett also provides an API that can be used to integrate other applications with torrents, such as a media center or home automation system.
Jackett is used as a proxy server for Sonarr and Cie. By itself it does not query or download files.