18. A system for delivering content from a corpus of data including a plurality of items, comprising: means for identifying a rule having a trigger that specifies a trigger query, and defines a content presentation action to be performed responsively to the trigger; means for receiving a search query and generating a search query result set by searching a corpus of data for items that satisfy the search query; means for generating a trigger query result set by searching the corpus of data for items that satisfy the trigger query; and means for presenting content corresponding to the content presentation action when the level of intersection between the user query result set and the trigger query result set is greater than a predetermined threshold.
18. A system for delivering content from a corpus of data including a plurality of items, comprising: means for identifying a rule having a trigger that specifies a trigger query, and defines a content presentation action to be performed responsively to the trigger; means for receiving a search query and generating a search query result set by searching a corpus of data for items that satisfy the search query; means for generating a trigger query result set by searching the corpus of data for items that satisfy the trigger query; and means for presenting content corresponding to the content presentation action when the level of intersection between the user query result set and the trigger query result set is greater than a predetermined threshold. 21. The system of claim 18 , wherein said level of intersection is determined by dividing the number of items in the intersection of the search query results and the trigger query results sets by the number of items in the trigger query result set.