What is Plex?
Plex is actually a couple of applications for home theater. Plex Media Server is run on a computer that will handle the serving of content, while various other Plex applications will display the content. There are Plex apps for Android and iPhone, Apple TV, Windows, Mac OS X, and Linux. Even the venerable Raspberry Pi family of devices can be used as a Plex client, making it a low-power option to hook up to your television. The beauty of the Plex system is that from one server, content can be displayed and synchronized between many devices.
Setup of the Plex media server, and access to some media is a prerequisite to using Plex. Up to date documentation on installing the Plex media server is available here. Once installed, you can add content of various types by pointing the Plex server to the locations where the files are. If the files are named correctly, Plex will automatically provide metadata and library functionality and present this information in a beautiful, clean interface.
Once the server is up and running, it will automatically be detected by client apps on the same local network. If you sign up for a Plex Pass, and if your server is reachable from the Internet (e.g., in a DMZ on your router), then you should also be able to get at the server's content from outside your network.
If you are like me, however, with a DSL modem and are cut off from the outside world by your ISP, then a valuable Plex Pass feature is Sync. Sync allows you to download to your Android or IOS device content from the Plex server for consumption later.
All in all, we love Plex for its flexibility and reliability. It has the ability to handle pretty much any file format you throw at it, and can transcode that content into the preferred format for any client. That such a software package is available for free and can be easily used by anyone is truly a boon to this media age!