How come some site urls do not include a file extension?

Question

I was browsing the internet and noticed, YouTube, for example, contains a URL like this to denote a video page: http://www.youtube.com/watch?v=gwS1tGLB0vc.

Answer 1

"www.youtube.com/watch" is a directory of YouTube. So it can basically be written as "www.youtube.com/watch/" with the ending forward slash.