The physical directory structure and names of your files can have an impact on the success of your Page Rank and keyword success.

• Whether it helps or not to put your keywords in your URLs is a debatable point. If you’re doing a logically organized, SEO-oriented site, each page is going to be associated with a main key phrase regarding what that page is about. It makes sense to carry that over to the URL name as it makes it much easier to mentally associate the content and the URL name so you can find whatever page it is that you need to deal with.
• Google wants to follow patterns of (natural) human nature for showing us results, so this would only be logical to make sure that your file names and directories make sense when organizing your material in your site.
• On the contrary, artificially inflating your file names with your keywords would not be a good idea (not natural), and might do harm to your rankings.
• Always use hyphens and not underscores in your URLs. This includes files names and directories. The reason why is because Google disregards underscores (_) but interprets hyphens (-) as a space.
• Depth of Directories – very deep pages tend to rank less well, or get less traffic. This is generally because:
  • All other things being equal, a web page in the root of your web site is seen as more important as a web page nested a folder or more deep.
  • Deep pages are more specific and therefore tend to grab only very specific subsets of larger searches
  • Deep pages tend to receive less links, link pop, and Page Rank (in Google’s case)