1. A computer-implemented method comprising: receiving, from a content sponsor, an indication of a first collection of entities to be used as selection criteria for presenting a first content item of a campaign responsive to received requests, wherein the first collection of entities is a grouping of entities that share at least one common characteristic; storing in inventory a reference to the first content item and the indicated selection criteria; after the storing, receiving a query including one or more terms or phrases, wherein the one or more terms or phrases identifies or is associated with a first entity; determining, using one or more processors, that the first entity is included in the first collection of entities, wherein the one or more terms or phrases does not include the first collection; identifying one or more eligible content items from an inventory of content items, each eligible content item being associated with selection criteria including criteria specifying the first collection of entities, wherein identifying includes identifying the first content item; and providing at least a portion of the first collection of entities for presentation to a user along with search results responsive to the query, wherein providing at least a portion of the first collection of entities includes providing one or more of the eligible content items including the first content item along with the portion of the first collection of entities including dynamically arranging a presentation of the portion of the first collection to position the first content item more prominently or visually emphasize the first content item based on a bid by the content sponsor.
1. A computer-implemented method comprising: receiving, from a content sponsor, an indication of a first collection of entities to be used as selection criteria for presenting a first content item of a campaign responsive to received requests, wherein the first collection of entities is a grouping of entities that share at least one common characteristic; storing in inventory a reference to the first content item and the indicated selection criteria; after the storing, receiving a query including one or more terms or phrases, wherein the one or more terms or phrases identifies or is associated with a first entity; determining, using one or more processors, that the first entity is included in the first collection of entities, wherein the one or more terms or phrases does not include the first collection; identifying one or more eligible content items from an inventory of content items, each eligible content item being associated with selection criteria including criteria specifying the first collection of entities, wherein identifying includes identifying the first content item; and providing at least a portion of the first collection of entities for presentation to a user along with search results responsive to the query, wherein providing at least a portion of the first collection of entities includes providing one or more of the eligible content items including the first content item along with the portion of the first collection of entities including dynamically arranging a presentation of the portion of the first collection to position the first content item more prominently or visually emphasize the first content item based on a bid by the content sponsor. 10. The method of claim 1 wherein the one or more eligible content items are advertisements and the first collection of entities is a group of entities that are distinct from the search results.