Master your home media setup with the Plex app for iOS from Apple iTunes!
Plex, at it's core, is a program to set up a DLNA server. Basically that means that you can install it on a computer and the media on that computer can be shared out to other DLNA enabled devices like other computers, PS3, Roku, etc... Depending on which model you have you can even stream video, audio or photos directly to your TV. Plex will also add cover art and proper titles to your movies and songs so if you acquired them completely legally and they came with weird names, you no longer have to go in a rename them all, Plex figures out what it is for you. Check out Plex for more details.
The Plex app is what let's you watch stuff from your Plex server on your phone or tablet. It also lets you stream stuff from 'Plex Channels' like Revision3, Discovery TestTube, TWiT and a bunch of others. It's all stuff you can get online but it just makes it easier and brings it all in to one place.
The Plex server is free so they make their money off of selling premium subscriptions and selling the app. That's why it's normally so expensive at $4.99. Right now you get the app for only $1.99. It's a great opportunity to try a great service and make managing your media library just a bit easier.