If you want to edit videos on your Windows Phone then there aren't too many decent options but Movie Maker 8.1 offers very good basic video editing tool.
Movie Maker 8.1 is very simple to use. Simply select a video or image from your phone's library, choose an audio file and then use the timeline to sync and cut the movie. You can also add titles, basic effects and 3D transitions to your movies.
The timeline is zoomed-in by default and there's no way to zoom-out and you can't drag the audio track but overall, Movie Maker 8.1 is a nifty little video editor for Windows Phones.