Trailers - How do I get them to work?

Hi there...

How do trailers work? I only just found out you can scrape trailers... I hear you can either have a URL or download a file into the folder with "-trailer" on the extension.

I think I would prefer to use a URL to save on some space... so how dose this work.. dose it use youtube or something? How do I get trailers working?

Thanks in advance