25. A computer program product, encoded on a non-transitory computer-readable storage device, operable 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 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 segments 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.
25. A computer program product, encoded on a non-transitory computer-readable storage device, operable 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 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 segments 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. 26. The product of claim 25 , wherein clustering the user actions into the series of segments using the temporal criteria comprises: identifying segmentation boundaries in the content access history, each segmentation boundary determined by an elapsed time period between two consecutive user actions, a length of the elapsed time period satisfying a boundary threshold; and clustering the user actions into the series of segments according to the segmentation boundaries.