The URL is the file name for the web page and is displayed in the web browser window. This is an example from my website: http://be-found.net/seo-tools/, and http://be-found.net/search-marketing-expert/ . Those URLs would be considered search friendly.
These would be considered unfriendly URLs: http://www.stuffkidslike.com/cgi-bin/category.cgi?category=melissa__doug and http://www.etsy.com/view_listing.php?listing_id=25929346. The data after the domain name is called a query string., the string of information tacked onto the end of a URL after a question mark.
