16. A system comprising: a tangible memory to store a plurality of instructions; and a processor to execute instructions in the memory to: extract, from a document, of a plurality of documents, a keyword candidate and context information associated with the document, and where the keyword candidate is selected from words included in both the document and in one or more previously received search queries, map, based on the context information, the document to a category, of a plurality of categories, calculate a respective number of occurrences of the keyword candidate in each of the plurality of documents, calculate, based on the respective number of occurrences of the keyword candidate in each of the plurality of documents, a first frequency and a second frequency, where the first frequency corresponds to the occurrences of the keyword candidate in a first subset of the plurality of documents associated with the category to which the document is mapped, and where the second frequency corresponds to occurrences of the keyword candidate in a second subset of the plurality of documents not associated with the category to which the document is mapped, determine that the keyword candidate is a keyword for the category based on the calculated first frequency and the calculated second frequency, and associate the keyword with the document in response to determining that the keyword candidate is the keyword for the category to which the document is mapped.
16. A system comprising: a tangible memory to store a plurality of instructions; and a processor to execute instructions in the memory to: extract, from a document, of a plurality of documents, a keyword candidate and context information associated with the document, and where the keyword candidate is selected from words included in both the document and in one or more previously received search queries, map, based on the context information, the document to a category, of a plurality of categories, calculate a respective number of occurrences of the keyword candidate in each of the plurality of documents, calculate, based on the respective number of occurrences of the keyword candidate in each of the plurality of documents, a first frequency and a second frequency, where the first frequency corresponds to the occurrences of the keyword candidate in a first subset of the plurality of documents associated with the category to which the document is mapped, and where the second frequency corresponds to occurrences of the keyword candidate in a second subset of the plurality of documents not associated with the category to which the document is mapped, determine that the keyword candidate is a keyword for the category based on the calculated first frequency and the calculated second frequency, and associate the keyword with the document in response to determining that the keyword candidate is the keyword for the category to which the document is mapped. 17. The system of claim 16 , where the processor, when determining that the keyword candidate is the keyword for category, is further to: determine that the first frequency, associated with the occurrences of keyword candidate in the first subset, of the plurality of documents, associated with the category to which the document is mapped, is greater than a threshold value, and determine that the second frequency, associated with the occurrences of keyword candidate in the second subset, of the plurality of documents, not associated with the category to which the document is mapped, is not greater than the threshold value.