Plex is a free media center app that allows you to store video on one device and stream it on others. That's not the only thing it does, of course. It also lets you share with friends, while encrypting everything securely. It lets you start a stream and then move it to another app, if that suits your fancy. In fact, it does pretty much everything.
Feature packed to the gills
Beyond just watching movies and videos you already have, Plex provides access to free, online television right from the app. For videos you own yourself, you can use the app to optimize them for better streaming. Past all that, it regularly sees new features. As of this writing, its DVR and Cloud features are both new. The former lets you record the free TV channels that Plex already provides and watch their contents later, while the latter makes your life easier by using Amazon as a streaming hub.
For all your media needs
Basically, get Plex if you want to have your movies stored on a computer far upstairs and watch them on a media center hooked up to the TV two floors below. Or if you just want to enjoy some online television. The app itself is well-designed, so if it has features you want, consider it.