HTTP cookies gave web applications a small client-side mechanism for carrying state across otherwise separate requests, enabling shopping carts and login sessions while also opening the door to cross-site tracking.
Tim Berners-Lee's Web joined universal identifiers, hypertext, a simple transfer protocol and portable document markup into an open information system that could spread across the Internet.
The early Web worked because independently built clients and servers could agree on three things: how to identify resources, how to request representations, and how to encode linked documents.
Roy Fielding's REST described the architectural constraints behind the scalable Web and later gave developers a vocabulary for designing network APIs around resources, representations and uniform interfaces.