Absolute (Universal Resource Locator) URL – an entire url address including the http:// or https:// and root of the address. For example http://www.utexas.edu/news.
Relative (Universal Resource Locator) URL – When you’re creating links on a webpage to another page on the same website you should not include the root part of the address. That way if you ever move your site or if you have a test version of your site with a different root, your links will still work. For example: /news.
Get the details and see examples in the article Absolute vs. Relative Paths/Links.