13. A system comprising: at least one processing unit; and one or more physical storage media storing computer-executable instructions which, when executed by the at least one processing unit, cause the at least one processing unit to: represent historical user behaviors of a user using a set of features, the set of features including click features and query features; train a classifier using training data comprising values for the features to identify a plurality of historical search contexts, wherein the plurality of historical search contexts include: a first historical search context having first values for the features, wherein the first values include: first click feature values for the click features, the first click feature values representing multiple first uniform resource locators (URLs) clicked on by the user as part of the first historical search context, and first query feature values for the query features, the first query feature values representing multiple first queries entered by the user as part of the first historical search context, a second historical search context having second values for the features, wherein the second values include: second click feature values for the click features, the second click feature values representing multiple second URLs clicked on by the user as part of the second historical search context, and second query feature values for the query features, the second query feature values representing multiple second queries entered by the user as part of the second historical search context; represent current user behavior during a current user session using third values for the features, wherein the third values include current click feature values for the click features and current query feature values for the query features, the current click feature values representing multiple current URLs clicked on by the user during a current session and the current query feature values representing multiple current queries entered by the user during the current session; during the current user session, determine that the current user behavior is relatively more similar to the first historical search context than the second historical search context, wherein the determining comprises analyzing similarity of: the current click feature values to the first click feature values and the second click feature values, and the current query feature values to the first query feature values and the second query feature values; and surface the first historical search context as part of the current session for the user.
20. The system of claim 13 , wherein the computer-executable instructions further cause the processing unit to: iteratively train the classifier to recognize additional historical search contexts based on subsequent training data obtained as the user enters subsequent clicks during subsequent user sessions.