Use the "[youtube ]video#"[/youtube ]" (omit spaces) convention. For example, for the http://www.youtube.com/watch?v=Tk-0FUKhhvU video copy video code after the v= in the url and put it between the "[ youtube] and [ /youtube]" tags.
Vimeo also added, usage is the same as youtube but but with "vv" instead of "youtube" in the brackets.
Also added are youtube and vimeo icons in the editor (if you forget).
Html code is not permitted due to the risk of malicious code being posted.