What are Relative URLS ?
When a web browser reads an HTML document, it has a great deal of information about the document. This includes the protocol used to retrieve the document, the name of the host where the document lives, and the path to that document on the host. Most of this is likely to be the same for many of the URLs in that document. Relative URLs inherit the protocol, hostname, and path of their parent document rather than respecifying it in each HREF attribute. Thus if any piece of the URL is missing, it is assumed to be the same as that of the document in which the URL is found. Such a URL is called a relative URL. In contrast, a completely specified URL is called an absolute URL.
the FAQ
If the relative link begins with a /, then it is relative to the document root instead of relative to the current file.
Relative URLs have a number of advantages. First and least they save a little typing. More importantly relative URLs allow entire trees of HTML documents to be moved or copied from one site to another without breaking all the internal links.