No,
PLEX is a media server/player--I believe it's a fork of XBMC. I began using it years ago as the media interface on my Mac Mini that I was using as a HTPC. It's functionality as grown over the years. In it's current iteration, it allows for serving of your media from a home 'server' (another Mac Mini in my case) to any device that runs the PLEX media player (Plex app on mobile devices or Roku, another computer, jailbroken
TVs, and I believe some SMART TVs).
I love it because it can playback nearly any codec and transcodes on the fly as needed for streaming to your playback device.
Having it now work with Chromecast means I can just grab the Chromecast when I travel, plug it into an HDMI port on a television and use my Android tablet or phone to stream anything from my personal media library using a local wifi network.
I'm fairly certain my description isn't entirely technically accurate but should cover the gist of it. All I know is it's going to make my week at the in-laws in Buffalo quite a bit more tolerable.