16. A system comprising: a tangible computer readable medium storing instructions that when executed by one or more processors cause the system to: determine top phrases for a plurality of documents in a limited document collection, wherein determining the top phrases of a document includes: identifying phrases in the document; for each identified phrase in the document, determining an importance score for the identified phrase based on occurrences of related phrases of the identified phrase, which are also in the document; associate each top phrase with the importance score for the document; for each top phrase, determine an aggregate score of the top phrase for the limited document collection based on the top phrase's scores for individual documents of the limited document collection in which the top phrase appears; and select a set of top phrases with the highest aggregate scores; and one or more processors configured for executing the instructions stored on the computer readable storage medium.
16. A system comprising: a tangible computer readable medium storing instructions that when executed by one or more processors cause the system to: determine top phrases for a plurality of documents in a limited document collection, wherein determining the top phrases of a document includes: identifying phrases in the document; for each identified phrase in the document, determining an importance score for the identified phrase based on occurrences of related phrases of the identified phrase, which are also in the document; associate each top phrase with the importance score for the document; for each top phrase, determine an aggregate score of the top phrase for the limited document collection based on the top phrase's scores for individual documents of the limited document collection in which the top phrase appears; and select a set of top phrases with the highest aggregate scores; and one or more processors configured for executing the instructions stored on the computer readable storage medium. 19. The system of claim 16 , wherein determining an aggregate score for the top phrase comprises summing the scores of the top phrase for each document for which the top phrase is a top phrase.