On today’s Internet, database driven or dynamic sites are very popular. Unfortunately the easiest way to pass information between your pages is with a query string. In case you don’t know what a query string is, it's a string of information tacked onto the end of a URL after a question mark.
So, what’s the problem with that? Well, most search engines (with a few exceptions - namely Google) will not index any pages that have a question mark or other character (like an ampersand or equals sign) in the URL. So all of those popular dynamic sites out there aren’t being indexed - and what good is a site if no one can find it?
The solution? Search engine friendly URLs. There are a few popular ways to pass information to your pages without the use of a query string, so that search engines will still index those individual pages.
Comparing Search Engine Friendly URLs to Parameter Clogs