Horizon is an iOS app that lets you record horizontal videos no matter how the device is held. This means that the videos will look good even if you record in portrait mode.
When you use the standard video app that comes with the iPhone or iPad and record in portrait mode, these black bars get added to the side and there is a reduced viewing area.
Horizon also has filters and works with the front and back cameras. Horizon also auto-levels the videos while recording using the gyroscope built into the phone. You can download the Horizon app for $0.99 on the Apple App Store.