1. A method of profiling a user of a computing device, connected to a network, based upon a user's on-line actions, the method comprising: receiving a user event generated by actions performed by the user on the computing device, the user event identifying event identification information indicating content requested by the user and a user identifier; performing a look-up in a cached web map using the event identification information to determine classification information associated with the content requested by the user, the classification information providing a text-score record comprising at least a text string and a score associated with the text string, the text string defined in relation to a lexical ontology comprising a hierarchy of categories, and the score representing a strength of association of the text string to the content; retrieving classification information associated with the event identification information from the cached web map if classification information is present for the event identification information; retrieving a user profile associated with the user identification information from a profile database; generating profile update information by applying profiling rules defining known associations between user action events and user behaviours to the retrieved classification information for the user event, wherein the profile update information identifies how to modify the user profile by adding classification identification information or modifying existing classification identification information scores stored in the profile; updating the user profile based upon the update information, the user profile providing a characterization of the user's preferences; and storing the updated user profile wherein the user profile can be accessed by an external advertising server.