A relative URI (sometimes called a relative link) is a partial URI that is interpreted (resolved) relative to a base URI.

On the World Wide Web, relative URIs come in two forms:

A relative URI with an absolute path is interpreted relative to the domain root:

/images/icecream.jpg → http://domain.example/images/icecream.jpg

A relative URI with a relative path is interpreted relative to the URL of the current document. E.g., on the web page http://domain.example/icecream/chocolate.html,

strawberry.html → http://domain.example/icecream/strawberry.html

Wikipedia: URI Resolution

Reference http://codex.wordpress.org/Glossary