URL Structure
Your website should try to keep URL’s simple. Best-case scenario you want to have your keywords in the URL’s and use dashes for spaces when it is a keyword phrase. Google recommends using dashes instead of underscores. This is a best practice but won’t affect your ranking unless people start to link to you incorrectly because they don’t see the underscores. The worst case is when you have URL’s that have user id’s. This will give the search engine spiders a different URL ever time they visit. This will create duplicate content in the search engines. If you have to use dynamic URL’s it is best to keep the parameters down to 2 and make sure everybody gets the same URL every time they visit. Some people recommend not using extensions (ex. .html, .asp, .php) because if you want to change technologies in the future it will be easier. They recommend URL’s like http://www.domain.com/subdirectory/page-title/
