it's [ youtube ] whatever is after the equal sign [ /youtube ]
ie take IVZMnai5CNE from http://www.youtube.com/watch?v=IVZMnai5CNE and embed it between the tags and you get
[youtube]IVZMnai5CNE[/youtube]
if the above works it means you can simply just paste then link to the video:
http://www.youtube.com/watch?v=IVZMnai5CNE and wrap it in the youtube tags and it will work