10. A non-transitory computer-readable volatile or non-volatile storage medium storing instructions which, when executed by one or more processors, cause the one or more processors to perform: maintaining one or more search engine indexes, associated with a search engine, that index collection items and base items; wherein each of the collection items: represents a collection of base items, and is treated, within the one or more search engine indexes, as an entity distinct from the base items represented by the collection item; wherein the collection items indexed in the one or more search engine indexes include a first collection item; storing, in a search engine index, of the one or more search engine indexes, a first index entry for the first collection item; wherein the first index entry includes a tag describing the first collection item itself, rather than for any of the base items represented by the first collection item; wherein the tag describing the first collection item is metadata that includes one or more of: a) one or more keywords that identify a plurality of base items in a collection that is represented by the first collection item, b) one or more keywords about the creator of the first collection item, or c) one or more keywords that describe the first collection item; storing data that indicates which items, indexed in the one or more search engine indexes, are collection items; in response to receiving a search query: searching the one or more search engine indexes for collection items and base items that match the search query; and identifying a set of matching items by determining that search query matches the first collection item by using the search engine index to compare the search query to the first index entry that is stored in the search engine index; wherein the set of matching items includes one or more particular base items and the first collection item; determining that the first collection item is a collection item based on the data that indicates which items, indexed in the one or more search engine indexes, are collection items; responding to said search query by generating search results based at least in part on the set of matching items, wherein the search results comprise: a) at least one of the one or more particular base items; and b) the first collection item; and wherein the method is performed by one or more computing devices.
10. A non-transitory computer-readable volatile or non-volatile storage medium storing instructions which, when executed by one or more processors, cause the one or more processors to perform: maintaining one or more search engine indexes, associated with a search engine, that index collection items and base items; wherein each of the collection items: represents a collection of base items, and is treated, within the one or more search engine indexes, as an entity distinct from the base items represented by the collection item; wherein the collection items indexed in the one or more search engine indexes include a first collection item; storing, in a search engine index, of the one or more search engine indexes, a first index entry for the first collection item; wherein the first index entry includes a tag describing the first collection item itself, rather than for any of the base items represented by the first collection item; wherein the tag describing the first collection item is metadata that includes one or more of: a) one or more keywords that identify a plurality of base items in a collection that is represented by the first collection item, b) one or more keywords about the creator of the first collection item, or c) one or more keywords that describe the first collection item; storing data that indicates which items, indexed in the one or more search engine indexes, are collection items; in response to receiving a search query: searching the one or more search engine indexes for collection items and base items that match the search query; and identifying a set of matching items by determining that search query matches the first collection item by using the search engine index to compare the search query to the first index entry that is stored in the search engine index; wherein the set of matching items includes one or more particular base items and the first collection item; determining that the first collection item is a collection item based on the data that indicates which items, indexed in the one or more search engine indexes, are collection items; responding to said search query by generating search results based at least in part on the set of matching items, wherein the search results comprise: a) at least one of the one or more particular base items; and b) the first collection item; and wherein the method is performed by one or more computing devices. 12. The non-transitory computer-readable volatile or non-volatile storage medium of claim 10 , wherein storing the index entry is performed in response to receiving a registration request from a service provider or program that is used to generate collection items.