The URL structure defines a hierarchy (or relationship) that is similar to the hierarchy of subdirectories (or folders) in the filesystems used by most computer operating systems. The segments of a URL are separated by slash characters ("/"). When navigating the URL hierarchy, the final segment of the URL (i.e., everything after the final slash) is similar to a file in a filesystem. The other segments of the URL are similar to the subdirectories and folders in a filesystem. 
A relative URL omits some of the information needed to locate the referenced document. The omitted information is assumed to be the same as for the base document that contains the relative URL. This reduces the length of the URLs needed to refer to related documents, and allows document trees to be accessed via multiple access schemes (e.g., "file", "http", and "ftp") or to be moved without changing any of the embedded URLs in those documents. 
Before the browser can use a relative URL, it must resolve the relative URL to produce an absolute URL. If the relative URL begins with a double slash (e.g., //, then it will inherit only the base URL's scheme. If the relative URL begins with a single slash (e.g., /faq/html/), then it will inherit the base URL's scheme and network location. 
If the relative URL does not begin with a slash (e.g., all.html , ./all.html or ../html/), then it has a relative path and is resolved as follows.

1. The browser strips everything after the last slash in the base document's URL and appends the relative URL to the result.
2. Each "." segment is deleted (e.g., ./all.html is the same as all.html, and ./ refers to the current "directory" level in the URL hierarchy).
3. Each ".." segment moves up one level in the URL hierarchy; the ".." segment is removed, along with the segment that precedes it (e.g., foo/../all.html is the same as all.html, and ../ refers to the parent "directory" level in the URL hierarchy). 

Some examples may help make this clear. If the base document is <URL:>, then

all.html and ./all.html
refer to <URL:> 
refers to <URL:> 
refers to <URL:> 
refers to <URL:> 
refers to <URL:> 

Please note that the browser resolves relative URLs, not the server. The server sees only the resulting absolute URL. Also, relative URLs navigate the URL hierarchy. The relationship (if any) between the URL hierarchy and the server's filesystem hierarchy is irrelevant.