11. One or more computer-storage media storing thereon a content-to-key phrase index data structure, the content-to-key phrase index data structure comprising: a first field for content identifiers that are associate with key phrases; a second field for key phrases derived from queries, in a search engine query log, provided by one or more users a third field for scores associated with each key phrase, the scores quantifying relationships between the key phrases and the content associated with the content identifiers, wherein scores are augmented based on a rank assigned by a search engine that returned the content in response to the queries and user interactions, including dwell times, with the content; and a fourth field for categories associated with each key phrase, the categories identifying relationships between the key phrases and the content associated with the content identifiers, wherein categories include dominant, seasonal, stable, tied-to-content, or representative-of-content.
11. One or more computer-storage media storing thereon a content-to-key phrase index data structure, the content-to-key phrase index data structure comprising: a first field for content identifiers that are associate with key phrases; a second field for key phrases derived from queries, in a search engine query log, provided by one or more users a third field for scores associated with each key phrase, the scores quantifying relationships between the key phrases and the content associated with the content identifiers, wherein scores are augmented based on a rank assigned by a search engine that returned the content in response to the queries and user interactions, including dwell times, with the content; and a fourth field for categories associated with each key phrase, the categories identifying relationships between the key phrases and the content associated with the content identifiers, wherein categories include dominant, seasonal, stable, tied-to-content, or representative-of-content. 15. The media of claim 11 , wherein the key phrases are stored in a tied-to-content list based on semantic uniqueness and the scores associated with the key phrases.