1. A method of classifying user activities, comprising: at a computer system having one or more processors and memory: receiving eye tracking data for a person interacting with a computer using one or more user interface devices, the eye tracking data comprising a sequence of fixations ordered temporally, wherein each respective fixation corresponds to a plurality of respective consecutive measured gaze points, and wherein each respective fixation has a respective duration and location based on the corresponding respective measured gaze points; receiving user interaction data from the one or more user interface devices; and for each fixation in the sequence: determining a plurality of features for the fixation, including one or more characteristics of the fixation and one or more user interaction features based on the received user interaction data; and associating a user activity label with the fixation in accordance with the plurality of features, wherein the user activity label is selected from a predefined set of user activity labels.