What is a REST API? The style behind most cloud services

REST is a style of API, not a different thing from an API

An API is simply the contract that lets one program ask something of another. REST describes how to design that contract for the web: model everything you can act on as a resource, give each resource its own address, and use a small, standard set of verbs to act on it. Two teams who have never spoken can still build compatible systems, because both are following the same well-known conventions rather than inventing their own.

That predictability is the whole point. A developer who has used one REST API can guess a great deal about a new one before reading a line of documentation, because the shape of the interaction — resources, methods, status codes — stays the same from service to service.

The core REST principles, plainly

A handful of ideas make an API "RESTful":

• Resources identified by URLs — a customer, an order or a file each has its own address, such as /orders/482, rather than one endpoint that does everything.
• Standard HTTP methods carry the meaning — GET reads a resource, POST creates one, PUT or PATCH updates one, DELETE removes one. The verb, not the URL, says what you are doing.
• Statelessness — each request carries everything the server needs to handle it (including who is calling, via a token or key); the server keeps no memory of previous requests from that client.
• Structured responses and status codes — replies are typically JSON, and a status code such as 200, 404 or 500 tells the caller what happened without it having to parse the body first.

Why REST became the default

REST won out largely because it reuses infrastructure the web already had. It runs over plain HTTP, so existing load balancers, proxies and caches understand it without modification; GET requests can be cached exactly like a web page. Statelessness is what makes it scale so easily — because no server needs to remember a client between requests, you can add more servers behind a load balancer and route any request to any of them. None of this required inventing new protocols, which is a large part of why REST spread so widely, so quickly.

REST versus the alternatives, briefly

REST is not the only API style you will meet. GraphQL lets a client specify exactly the fields it wants in a single query, which suits complex or deeply nested data — our explainer on what GraphQL is covers the trade-off against REST in full. gRPC is a high-performance style built for fast service-to-service calls inside a system, less commonly exposed to outside clients. For a beginner, the practical takeaway is simpler than picking sides: REST is what you will encounter first, most often, and by default on cloud exams.

REST in the cloud

Cloud providers run on REST APIs. Every action you take through a web console or command-line tool is translated into a REST call against the provider’s own API underneath — creating a virtual machine, writing a database record or updating a DNS entry are all, ultimately, HTTP requests to a resource-shaped endpoint. When you build and expose your own REST APIs, an API gateway typically sits in front to handle authentication, rate limiting and routing: Amazon API Gateway on AWS, Azure API Management on Azure, and Google Cloud API Gateway on Google Cloud all fill this role.

REST in certification study

REST literacy sits squarely in developer-focused exam content. The AWS Developer Associate exam expects you to design and secure APIs with API Gateway, understand HTTP methods and status codes, and reason about statelessness when an application scales. The Solutions Architect Associate treats REST APIs as connective tissue between services in a larger architecture. Cloud Practitioner needs only the concept — that cloud services are, underneath, resource-oriented APIs reached over HTTP.