rTorrent is a fast and efficient command-line based BitTorrent client. It can work many front-end interfaces such as ruTorrent. When used with a web interface it is similar to other clients like uTorrent Server or Deluge.
Jackett is a proxy server that provides a translation service between applications like CouchPotato, Sonarr or Headphones and tracker sites. It supports many private trackers. Instead of configuring the tracker directly in the app, it is now configured in the web interface and added to the app as a generic indexer.