plain URL on new line results in:

http://www.youtube.com/watch?v=pozXkBMkSYM

 

Inside a paragrah a youtube URL will just be treated as text  http://www.youtube.com/watch?v=pozXkBMkSYM, usually.

 

For finer control use the ‘Embed ‘ shortcode eg.

http://www.youtube.com/watch?v=pozXkBMkSYM