16. A method comprising: identifying, by one or more processors and for a first document, a plurality of second documents that each include a link pointing to the first document; determining, by one or more processors, a second document of the plurality of second documents that has been used to access the first document more than any other second document of the plurality of second documents; extracting, by one or more processors, first concepts from the determined second document, at least one extracted first concept, of the extracted first concepts, being associated with anchor text in the determined second document; identifying, by one or more processors and for the first document, one or more advertisements included in the first document; extracting, by one or more processors, second concepts from the identified one or more advertisements; and associating, by one or more processors, the extracted first concepts and the extracted second concepts with the first document.
16. A method comprising: identifying, by one or more processors and for a first document, a plurality of second documents that each include a link pointing to the first document; determining, by one or more processors, a second document of the plurality of second documents that has been used to access the first document more than any other second document of the plurality of second documents; extracting, by one or more processors, first concepts from the determined second document, at least one extracted first concept, of the extracted first concepts, being associated with anchor text in the determined second document; identifying, by one or more processors and for the first document, one or more advertisements included in the first document; extracting, by one or more processors, second concepts from the identified one or more advertisements; and associating, by one or more processors, the extracted first concepts and the extracted second concepts with the first document. 18. The method of claim 16 , further comprising: labeling one or more of the extracted second concepts as relevant to the first document or useful to an audience of the first document.