How does this nav work

[mrsocks]

Hello,

I am wondering how this navigation works:
http://www.soundcity.com

If you rollover any of the main categories you see a URL of http://www.soundcity.com/category/X

I think that the way this works is by having a folder named "category" with an index page that explodes the URL and then uses the last element in the array as the variable.

But then how does this work:
http://www.soundcity.com/category/6/108/170

is the index page just checking for set variables or what?

I am assuming that this URL structure is setup for the benefit of search engine placement, but I woudl like to know how this is done.

Any advice/insight would be great.
Thank you.

[mtmosier]

http://www.evolt.org/article/Search_Engine_Friendly_URLs_with_PHP_and_Apache/17/15049/

[mrsocks]

so is this type of URL structure even worth doing anymore since the search search engines are now indexing sites with the full URL including info after the ?

Thank you.