It works on a Client/Server basis:
Plex Media Server organizes and catalogues your media (movies, tv series, music etc), and automatically downloads cover art, description and related info. Plex Media Server can run on your computer (Mac, Windows or Linux) or NAS (Synology, QNAP and some others).
Plex client allows you to play the media stored on your Plex Media Server. There are Plex clients for Mac, Windows or Linux PCs (and it can run on the same PC as Plex Media Server or on a different one), for iOS, Android and Windows Phone devices, for some Media Players and for some LG and Samsung Smart-TVs.
There is even a Plex client for AppleTV but it only runs on Jailbroken AppleTVs. Well, not any more!
Some clever fellows noticed that some stock AppleTV apps are in fact just web-apps and download all content AND user-interface elements from Apple Servers. For instance, the Movie Trailers app on the AppleTV gets all it’s content from trailers.apple.com.
So, they made a small DNS Server that redirects all calls to trailers.apple.com to an app running on a local computer that masquerades itself as the Trailers App, reading media from Plex Media Server.
Then, all you have to do to use it is change the DNS network setting on your AppleTV. No jailbreak necessary. You just open the AppleTV Movie Trailers app and get Plex instead!
The app is called PlexConnect. It is still under development and you need Terminal to install and run it (essentially it’s a bunch of python scrips) but it shouldn’t be that hard. For more information, you can check their wiki.
I used to have a Mac Mini connected to my TV, running as a media center with Plex but then decided it was a shame to have a whole Mac Mini used just for watching movies. So I repurposed it, using it as our main home computer and media library, and just use iTunes and an AppleTV to play movies, tv series and music on my TV.
Now though, I think I’ll give Plex another chance.