17. A system comprising: one or more computers each including a processor and memory, the one or more computers configured to perform operations comprising: receiving a content access history of a user, the content access history including one or more user actions, each user action being associated with a content item upon which the user action is performed; identifying one or more action trails from the content access history, each action trail including a sequence of user actions corresponding to content items relating to a topic, wherein identifying a particular action trail includes: clustering the user actions into a series of segments using one or more temporal criteria; splitting a segment of the series of segments into multiple segments when a topical coherence value of the segment satisfies a threshold coherence value, wherein the topical coherence value of the segment measures a similarity between content items corresponding to user actions in the segment; calculating semantic similarities between the content items, wherein the semantic similarities change as a function of the series of segments; and adding one or more segment of the series of segments to the action trail, when the semantic similarities between content items corresponding to the user actions in the one or more segments and content items corresponding to the user actions in another segment satisfy a similarity threshold; and providing the action trails for display on a display device.
17. A system comprising: one or more computers each including a processor and memory, the one or more computers configured to perform operations comprising: receiving a content access history of a user, the content access history including one or more user actions, each user action being associated with a content item upon which the user action is performed; identifying one or more action trails from the content access history, each action trail including a sequence of user actions corresponding to content items relating to a topic, wherein identifying a particular action trail includes: clustering the user actions into a series of segments using one or more temporal criteria; splitting a segment of the series of segments into multiple segments when a topical coherence value of the segment satisfies a threshold coherence value, wherein the topical coherence value of the segment measures a similarity between content items corresponding to user actions in the segment; calculating semantic similarities between the content items, wherein the semantic similarities change as a function of the series of segments; and adding one or more segment of the series of segments to the action trail, when the semantic similarities between content items corresponding to the user actions in the one or more segments and content items corresponding to the user actions in another segment satisfy a similarity threshold; and providing the action trails for display on a display device. 19. The system of claim 17 , wherein calculating semantic similarities between the content items comprises calculating cosine similarities between topic vectors of the content items, the topic vectors of the content items each including a list of topics, each topic including a semantic descriptor of one or more of the content items.