http://help.youtube.com/support/youtube/bin/answer.py?answer=55744&topic=10526
Video Formats: File formats
YouTube accepts a wide range of video file formats such as .WMV, .AVI, .MOV, and .MPG transferred from most digital cameras, camcorders, and cell phones.
If you don’t think your current video file format is recognized by YouTube, you may get the best results from converting your file to MPEG4 video with MP3 audio.