1. Uniform Resource Identifier (URI)
http://en.wikipedia.org/wiki/Uniform_Resource_Identifier
2. Uniform Resource Locator (URL)
http://en.wikipedia.org/wiki/Uniform_Resource_Locator
3. Uniform Resource Name (URN)
http://en.wikipedia.org/wiki/Uniform_Resource_Name
4. Guidelines for URI Design
origin : http://css-tricks.com/guidelines-for-uri-design/
chinese : http://www.oschina.net/news/11105/url-knowledge
5. URL redirection
http://en.wikipedia.org/wiki/URL_redirection
6. Canonical URL Tag – The Most Important Advancement in SEO Practices Since Sitemaps
http://www.seomoz.org/blog/canonical-url-tag-the-most-important-advancement-in-seo-practices-since-sitemaps