URL, or Uniform Resource Locator, is a subset of the Uniform Resource Identifier (URI) that specifies where an identified resource is available and the mechanism for retrieving it.
This is an excellent cheat sheet that I wish I would have had 6 years ago when I purchased my domain. At the time I was not aware that subdomains would dilute domain authority. I did not know that keywords needed to be in the URLs and URIs and that URL length and keyword position affects the effectiveness of the Keywords. Well, we all live and learn.
As part of my self-hosted WordPress installation I use Yoast SEO. Since I started using it I’ve been able to focus my writing a little more clearly and get my META information, content and keywords to all agree. I went from being ignored on Google to having a Page Rank of 2 on http://garden.earthformed.com and 3 on http://blog.earthformed.com. This isn’t the only metric Google uses in their algorithm but it doesn’t hurt to have a grasp on how URLs are regarded by their system.
See on www.seomoz.org