I was not able to write anything better than the web site states its self. Here is the excerpt from the home page.
RCDefaultApp is a Mac OS X 10.2 or higher preference pane that allows a user to set the default application used for various URL schemes, file extensions, file types, MIME types, and Uniform Type Identifiers (or UTIs; MacOS 10.4 only). MacOS X uses the extension and file type settings to choose the application when opening a file in Finder, while Safari and other applications use the URL and MIME type settings at other times for content not related to a file (such as an unknown URL protocol, or a media stream).
RCDefaultApp also allows setting the application when digital cameras or FireWire webcams such as iSight are connected to the computer. For completeness’ sake, there are also settings that mimic the existing “CDs and DVDs” preference pane, which allows setting the action to take when various types of CDs and DVDs are inserted into the computer.
Finder’s Get Info panel can be used to set the default application for file extensions and file types, but it’s not intuitive. Apple’s Internet preference pane in MacOS X 10.0 through 10.2 (Jaguar) allowed you to set the Web and Email applications, but no other URL handlers. In MacOS X 10.3 (Panther), these settings have moved to Safari’s and Mail’s preferences — meaning you have to run those applications if you want to choose to not use them. Similarly, the digital camera preference is located in Image Capture’s preferences, and the webcam preference is a checkbox in iChat’s preferences. RCDefaultApp allows the setting of all of this information, and ties everything together in one preference pane.
RCDefaultApp also allows entries to be disabled, such as URL schemes that have security issues 😉