User Interface design patterns are used for common conventions like checkout, search, and navigation in web sites and web applications. They are typically used when you have a common and often recurring problem that needs to be solved. Design patterns exist to make code re-use and re-writing easy and ready to implement with minimal research or development time.
There are many resources like UIE or Nielsen Norman Group that contain accepted patterns for the user experience community. Smashing Magazine is also a great resource for design patterns and several UI pattern libraries are available as well: