Forced browsing is a technique used by attackers to gain access to resources that are not referenced, but are nevertheless accessible.
One technique is to manipulate the URL in the browser by deleting sections from the end until an unprotected directory is found. A related technique is to use a scanning tool like nikto to request common directories until a hidden file or directory is found.
Another technique is to use manual penetration testing to attempt access to resources that are not referenced in the application. For example, an attacker might use WebScarab to change request parameters that specify the target resource.