1. A method for customizing a user interface to an on-demand database service that is subscribed to by a plurality tenants, each having one or more users, the method comprising: receiving, from a first of a plurality of tenants, one or more definitions of a customization of a user interface, wherein at least one of the definitions includes a reference to content not stored in the on-demand database; associating the definitions with information that is specific to the first tenant and that is stored in the on-demand database; receiving, from a user of the first tenant, a request for at least a portion of the information; and sending, to the user, a user interface modified according to the definitions such that a customized user interface that includes a combination of the referenced content and the requested information is displayed to the user.