6. A system for selecting content for display on a user device via a computer network, comprising: an interface module of a data processing system having one or more processors configured to: receive, from a content provider associated with a content item, content selection criteria specifying types of queries that make the content item eligible to be selected for display along with search results responsive to queries matching the specified types of queries; and receive a search query provided via a user device; a query reference module of the data processing system configured to: determine via a data structure having information about entities, a content selection criteria entity associated with the content selection criteria and one or more related entities that do not appear in the content selection criteria but are related to the content selection criteria entity in the data structure; generate, using the data structure, a content selection criteria structure that includes an entry for the content selection criteria entity, an entry for each entity related to the content selection criteria entity, and a connection between each pair of related entities in the content selection criteria structure, the generated content selection criteria structure being separate from the data structure; determine, via the data structure, a search query entity, and one or more related entities that do not appear in the search query but are related, in the data structure, to the search query entity; generate a query structure that includes an entry for the search query entity, an entry for each entity related to the search query entity, and a connection between each pair of related entities in the query structure, the generated query structure being separate from the content selection criteria structure and the data structure; a matching module of the data processing system configured to determine a match between the content selection criteria structure and the query structure, including matching a topology and content of the content selection criteria structure to a topology and content of the query structure; and a content selector of the data processing system configured to select the content item as a candidate for display on the user device based on the match between the content selection criteria structure and the query structure.
6. A system for selecting content for display on a user device via a computer network, comprising: an interface module of a data processing system having one or more processors configured to: receive, from a content provider associated with a content item, content selection criteria specifying types of queries that make the content item eligible to be selected for display along with search results responsive to queries matching the specified types of queries; and receive a search query provided via a user device; a query reference module of the data processing system configured to: determine via a data structure having information about entities, a content selection criteria entity associated with the content selection criteria and one or more related entities that do not appear in the content selection criteria but are related to the content selection criteria entity in the data structure; generate, using the data structure, a content selection criteria structure that includes an entry for the content selection criteria entity, an entry for each entity related to the content selection criteria entity, and a connection between each pair of related entities in the content selection criteria structure, the generated content selection criteria structure being separate from the data structure; determine, via the data structure, a search query entity, and one or more related entities that do not appear in the search query but are related, in the data structure, to the search query entity; generate a query structure that includes an entry for the search query entity, an entry for each entity related to the search query entity, and a connection between each pair of related entities in the query structure, the generated query structure being separate from the content selection criteria structure and the data structure; a matching module of the data processing system configured to determine a match between the content selection criteria structure and the query structure, including matching a topology and content of the content selection criteria structure to a topology and content of the query structure; and a content selector of the data processing system configured to select the content item as a candidate for display on the user device based on the match between the content selection criteria structure and the query structure. 7. The system of claim 6 , wherein the data processing system is further configured to: translate properties of the search query entity into a flat data structure comprising information associated with the properties; and identify, using the flat data structure, a plurality of content selection criteria associated with the flat data structure.