Beginner question re Barlows

Other than the length of the tube, what is the significant difference between a Barlow and a shortly Barlow? I'm aware that in some cases use of a Barlow can make focusing a little more difficult but is there a distinct benefit from one to the other?

Joe

Here's a thread to read.

http://www.astromart.com/forums/viewpost.asp?forum_post_id=390223