What is a REST API, and what are its core architectural constraints?

REST (Representational State Transfer) is an architectural style for designing networked APIs around resources identified by URIs. Its core constraints are: client-server separation, statelessness (each request contains everything needed to process it), cacheability, a uniform interface (standard HTTP verbs acting on resources), a layered system, and optionally code-on-demand — together these make REST APIs scalable, simple to reason about, and independently evolvable on client and server sides.