1. A method comprising: identifying, by one or more computers, a corpus of events, each event including event information associated with an event associated with a user device; filtering, by the one or more computers, the corpus of events to identify, from among the corpus of events, a set of events, including multiple types of events, performed at a first user device within a single session in a specified time period, wherein the set of events includes multiple queries submitted at the first user device and one or more search results that were selected from respective search results pages presented at the first user device following submission of respective search queries; evaluating, by the one or more computers, the set of events as a whole, including: identifying, by the one or more computers, a set of terms that are included in the set of events, including: for each given event in the set of events, identifying, by the one or more computers, event-related terms that are included in event information included in the given event; and adding, by the one or more computers, the event-related terms identified for each event in the set of events to the set of terms that are included in the set of events; determining, by the one or more computers, a set of candidate keywords based on the identified terms in the set of terms, each candidate keyword including a different combination of terms from the set of terms identified from the set of events; determining, by the one or more computers and for each given candidate keyword in the set of candidate keywords, a term frequency of each term included in the given candidate keyword in the event information for the set of events; and determining, by the one or more computers, top ranked candidate keywords in the set of candidate keywords based on the determined term frequency of each of the terms included in each of the given candidate keywords; receiving, by the one or more computers, a request for content from the first user device; and selecting, by the one or more computers, a first content item using the determined top ranked candidate keywords, the first content item being different than the top ranked keywords.