18. A method for ranking discussion threads of a question-and-answer forum, each discussion thread having a question post and a plurality of answer posts, the method comprising: providing a query; receiving a query that includes terms; for each of a plurality of discussion threads, identifying features of the discussion thread, the features including a combined post feature representing a collection of terms from multiple answer posts of the plurality of answer posts of the discussion thread, a question feature representing a question of the discussion thread, a question post feature representing terms of a question post of the discussion thread, and one or more features selected from a group consisting of an answer feature, a popularity feature, an authority feature, an importance feature, an internal clickthrough feature, and an external clickthrough feature; generating a feature score for each of the identified features; and generating a relevance score for the discussion thread based on a weighted combination of the feature scores for the discussion thread wherein the relevance score for the discussion thread is an overall score representing relevance of the discussion thread to the query; and ranking the discussion threads based on the overall scores of the discussion threads.
18. A method for ranking discussion threads of a question-and-answer forum, each discussion thread having a question post and a plurality of answer posts, the method comprising: providing a query; receiving a query that includes terms; for each of a plurality of discussion threads, identifying features of the discussion thread, the features including a combined post feature representing a collection of terms from multiple answer posts of the plurality of answer posts of the discussion thread, a question feature representing a question of the discussion thread, a question post feature representing terms of a question post of the discussion thread, and one or more features selected from a group consisting of an answer feature, a popularity feature, an authority feature, an importance feature, an internal clickthrough feature, and an external clickthrough feature; generating a feature score for each of the identified features; and generating a relevance score for the discussion thread based on a weighted combination of the feature scores for the discussion thread wherein the relevance score for the discussion thread is an overall score representing relevance of the discussion thread to the query; and ranking the discussion threads based on the overall scores of the discussion threads. 24. The method of claim 18 further comprising outputting an indication of the ranking of the discussion threads.