Consumer Analytics - Privacy in the HTML context.
A user can be distinguished from another by the user's IP address. (IP addresses are not a perfectly match to a user due to routing, proxy, …);
Technologies such as onion routing can be used to further anonymize requests so that requests from a single user at one node on the Internet appear to come from many disparate parts of the network.
Cookies are designed specifically to track a user, and are the basis of most of the Web's session features.