The user has previously registered. When log-in again site reconfigures itself to preset preferences. Track and gather information about users behavior in temporary files called cookies. Personalized e-mail accounts. Content and layout configuration.
Users can select site screen layout and content sources based on interests.
Sites provide hard-disk storage space. These may notify you when product is in stock. Tailoring based on past user behavior. Tailoring based on behavior of other users with similar preferences. Personalization by user.