See: http://www.google.com/support/youtube/bin/answer.py?answer=183851
The Video Editor is a free tool in your YouTube Account which allows you to edit various clips and produce an entirely new edited video. Here’s what you can do with the Video Editor:
- Combine multiple videos you’ve uploaded to create a new longer video
- Trim at the beginning and / or ending of your videos
- Add a soundtrack from our AudioSwap library
- Create new videos without worrying about file formats, and publish the new video to YouTube with one click (no new upload is required)
Web 2.0: Who, What, Why, How 