1. A method for customizing a user interface to an on-demand database service that is subscribed to by a plurality of tenants, each tenant being an organization that subscribes to the on-demand database service and including a plurality of users, the method comprising: receiving, from a first of the plurality of tenants, first information specific to the first tenant; storing the first information in a database that stores other information received from other tenants, each of the other information specific to a particular tenant; receiving, from the first tenant, one or more definitions of a customization of a user interface for accessing the first information in the database, the definitions specific to the first tenant and each definition associated with a particular portion of the first information specific to the first tenant, wherein at least one of the definitions includes a reference to content stored at a server external to the on-demand database service, wherein the on-demand database service includes: one or more servers running an application for communicating with users of the tenants, the application providing access to the database; associating the definitions with the first information that is specific to the first tenant and that is stored in the database; receiving, at the application running on the one or more servers from a first user of the first tenant, a request for at least a portion of the first information specific to the first tenant; and in response to the request, sending, to the first user, a user interface modified according to the definitions associated with the at least a portion of the first information requested by the first user, such that a customized user interface that includes a combination of the referenced content and the requested information is displayed to the user for use by the first user in accessing the at least a portion of the first information requested by the first user; wherein the modified user interface is generated in response to the request from the first user by program code interacting with the definitions; wherein a first one of the definitions is for a language translation and is associated with web content of one or more third parties that is not stored utilizing the on-demand database, such that in response to a first request from the first user for the web content of the one of the third parties specified by a URL of the one of the third parties: sending, to the first user, the user interface modified according to the first one of the customizations including the language translation of the web content of the one of the third parties specified by the URL of the one of the third parties; wherein a second one of the definitions references a place of business and is associated with a map of the place of business provided by another one of the third parties, such that in response to a second request from the first user: sending, to the first user, the user interface modified according to the second one of the customizations by including the map of the place of business in the user interface.