Plex is the main interface for users. It indexes media in /srv/ext/media and lets everybody watch it whetever.
Sonarr lets you tell it what you'd like to have, and watches various feeds, waiting for it to appear. Then it tells a downloader to download it. Once a file is downloaded, it moves it into a directory where Plex can see it, making sure it's named properly.
Jackett is a feed provider, which searches torrent sites and provides the results as feeds. NZBGeek is a paid feed provider that I use: it monitors usenet for new posts, and provides them as feeds. Usenet is a very old distributed bulletin board: it even predates the World Wide Web. I pay Frugal Usenet $5 a month to run a Usenet server so I don't have to.
Transmission and NZBGet are downloaders. They pull files down from bit torrent (Transmission) and Usenet (NZBGet).