15. A computer program product, encoded on a non-transitory computer-readable storage device, configured to cause data processing apparatus 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 temporal proximity of the user actions; calculating semantic similarities between the content items, wherein the semantic similarities change as a function of the series of segments; and determining whether to add a segment of the series of segments to the action trail based on the semantic similarities between content items corresponding to the user actions in the segment and content items corresponding to the user actions in another segment; and providing the action trails for display on a display device.
15. A computer program product, encoded on a non-transitory computer-readable storage device, configured to cause data processing apparatus 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 temporal proximity of the user actions; calculating semantic similarities between the content items, wherein the semantic similarities change as a function of the series of segments; and determining whether to add a segment of the series of segments to the action trail based on the semantic similarities between content items corresponding to the user actions in the segment and content items corresponding to the user actions in another segment; and providing the action trails for display on a display device. 20. The computer program product of claim 15 , wherein determining whether to add a segment of the series of segments to the action trail based on the semantic similarities between content items corresponding to the user actions in the segment and content items corresponding to the user actions in another segment comprises: determining a semantic similarity between (i) content items corresponding to user actions in the segment and (ii) content items corresponding to user actions in the other segment; and determining whether the semantic similarity satisfies a threshold.