A system, method, and various software products provide for consistent identification of web users across multiple web sites, servers and domains, monitoring and capture of data describing the users' web activities, categorization of the web activity data, aggregation of the data into time dependent models describing interest of users and groups over time. Categorization is made with respect to a category tree which may be standardized or customized for each web site. User groups may be defined based on membership rules for category interest information and demographics. Individual user profiles are then created for users automatically based on satisfaction of the user group membership rules. As new data is collected on a user over time, the category interest information extracted from the user's web activity is updated to form a current model of the user's interests relative to the various categories. This information is also used to automatically update group membership and user profile information. Identification of users across multiple sites is provided by a global service that recognizes each user and provides a globally unique identifier to a requesting web server, which can use the identifier to accumulate activity data for the user. Client side user identification is provided to track user activity data on web servers that do not communicate with the global service and do not process activity for category information. User profiles may be shared among web sites that form alliances. User activity data may be aggregated along various dimensions including users/user groups, categorization, and time to provide robust models of interest at any desired time scale.