Video Tuner is Microsoft's official video editor for Nokia Lumia Phones. Video Tuner allows you to perform simple edits such as cropping, adding soundtracks and adding filters to videos recorded on your Lumia.
Other edits possible include rotate, flip, mirror, change speed and adjustment of exposure and contrast.
Video Tuner is easy to use but the stability in general is poor. It tends to crash on startup and freeze during edits although fully updating your Lumia via Software Update helps.