What format is "undf"?

The 'undf' codec type for VLC just means that VLC did not recognize the format or, in other words, it is not one of the supported formats.

I would recommend using the GSpot Codec Information Appliance. This utility can identify just about any kind of media to help you find the right codec to play it.

undf is an undefined file mainly seen while opening partly downloaded files in VLC media player. The file is not yet build fully for any player to recognize the format. So wait for the file to dowload fully or till the stage where the player is able to recognize the format of the file. Be patient while downloading file.

Also if the file is corrupted we get an undf - undefined file.

It's an undefined file, either a file which is not complete or not designed for your operating system.

Usually downloaders from BitTorrent sites will download a movie and try to preview it and receive this error. Just wait till the download completes.