Firstly one needs to understand the difference between static and dynamic URLs.

Dynamic Pages

Dynamic website page names are used to send variables to a script file in order to perform a task that usually generates dynamic results. Dynamic URLs always contain question mark that separates file name from sent variables. Example name "do.php?a=1&b=2" would pass variable a=2 and b=2 to "do.php" script file that generates some response.

Such technology is very useful for web designers but it down performs in search engine listings. Search engines prefer static URLs that are less likely to change over time.

Static Pages

To explain static page naming we are going to use two example URLs: "about.htm" and "about.php".

Classic example of a static page name consists of name and extension. Page name extension indicates technology ("htm" says that page uses HTML technology) or a programming language ("php" extension indicates that page was generated using PHP server side scripting) used.

Many consider that pages with extension other than "htm" or "html" are worth more than those with server side scripting language indicators (like PHP, ASP and others).

If you will try to search for "search engine optimization" in Google and look at domain names of first 20 web sites, you will notice that URLs with many extensions are present. This indicates that search engines don't really care about extension as long as the page represents useful information for the visitor.

Friendly URLs

Friendly URLs are static page names with no extension at all. Friendly page names are very useful to the user as they are easy to remember (E.g. help, shop, contact …). Page name extension that refers to scripting language/technology used in the page. Extension-less construction protects page names from becoming obsolete if different web technology is going to be used to generate / present them in the future. Such names can be achieved only using URL rewriting technology.