What are REST APIs? An introduction

If you’ve explored modern web or application development, you’ve almost certainly come across the term REST API. REST APIs sit behind many of the digital products we use every day, enabling different systems to communicate, share data and work together reliably at scale.

This blog introduces REST APIs in plain language. It explains what they are, how they work, and why they have become such a fundamental part of modern web development.

Understanding APIs

An API, or Application Programming Interface, is a set of rules that allow one piece of software to communicate with another. APIs define what requests can be made, how those requests should be structured, and what responses will be returned.

A simple way to think about an API is to compare it to a car’s controls. A driver does not need to understand how the engine or transmission works internally; they interact with pedals, steering wheel and gear stick instead. In the same way, an API allows developers to use functionality or data from a system without needing to know how it is implemented internally.

In practice, APIs are used everywhere: from retrieving data from a database, to integrating third-party services, to connecting front-end interfaces with back-end systems.

What is REST?

REST stands for Representational State Transfer. It is not a technology or programming language, but an architectural style that defines a set of constraints for designing networked applications.

REST was originally introduced to help manage communication. It provides guidelines for how resources should be identified, requested and transferred between systems in a consistent and scalable way.

A REST API is therefore a web service built according to REST constraints. While a web application is typically designed for human users, a web service is designed for system-to-system communication. REST APIs enable different applications, often built by different organisations, to exchange information in a predictable and reliable way.

What are REST APIs used for?

REST APIs are widely used across many areas of software development. Common use cases include:

• Web and mobile application development, where front-end interfaces retrieve and update data from back-end systems
• Microservices architectures, where applications are split into smaller, independent services
• Cloud-based applications that need to scale efficiently
• Internet of Things (IoT) device communication
• Third-party integrations, such as payment providers or mapping services
• Data sharing between organisations

Because REST APIs are lightweight, flexible and widely supported, they have become the default choice for many modern systems.

How do REST APIs work?

At a basic level, a REST API works through a simple request-and-response pattern. A client, such as a web browser or mobile app, sends a request to a server when it needs a resource. The server processes that request and return a response.

This typically follows four steps. First, the client sends a request to the server using a format defined by the API documentation. Second, the server may authenticate the client and check that it is authorised to make the request. Third, the server processes the request internally. Finally, the server returns a response indicating whether the request was successful and, if appropriate, includes the requested data.

REST APIs use HTTP, the same protocol that powers the web. Each request includes a URI that identifies the resource, a HTTP method that defines the type of operation, headers that provide additional information, and sometimes a body containing data.

The HTTP method signals the intent of the request. For example, a GET request is used to retrieve data, while POST, PUT or PATCH requests are used to create or update resources, and DELETE requests are used to remove them. While REST APIs often interact with databases, the resources they expose do not have to map directly to database records.

The constraints of REST

For an API to be considered RESTful, it must follow a set of architectural constraints. These constraints are designed to improve consistency, scalability and reliability.

Client-server architecture

REST APIs separate the client from the server. In the case of a web app, the client is responsible for the user interface, while the server manages data and logic. This separation allows both sides to evolve independently.

Uniform interface

The API exposes a consistent and generic interface. It is not designed for a single client, but for any client that follows the rules. Resources are represented in a standard format, which may differ from how data is stored internally on the server.

Statelessness

Each request is independent. The server does not store information about previous requests, meaning every request must contain all the information needed to process it. This improves scalability and reliability.

Cacheability

Responses should be marked as cacheable or non-cacheable. Caching improves performance by reducing unnecessary requests and server load.

Layered system

REST APIs can be built from multiple layers, such as security, load balancing or business logic. These layers remain invisible to the client, which only interacts with the API endpoint.

Code on demand (optional)

In some cases, a server can send executable code to extend client functionality. This constraint is optional and not commonly used.

Benefits of REST APIs

REST APIs offer several advantages that explain their widespread adoption.

They are highly scalable, as statelessness reduces server overhead. They are flexible, allowing clients and servers to evolve independently. They are technology-agnostic, meaning clients and servers can be written in different programming languages. REST APIs are also relatively easy to use and lightweight compared with more rigid protocols.

REST APIs compared with other approaches

SOAP is an older web service protocol with strict standards and mandatory XML messaging. While it includes built-in security and transactional features, it is often heavier and slower than REST.

GraphQL is a newer API technology that allows clients to request exactly the data they need in a single query. This can reduce the number of requests required, but it also introduces additional complexity. REST remains simpler and more widely adopted, making it a common starting point for developers.

How to learn REST APIs

Learning REST APIs is an important step for anyone working in web or application development. A structured learning path helps you understand not just the theory, but also how APIs are designed, consumed and tested in real-world scenarios.

StayAhead offers dedicated training in APIs and web development, with flexible study options to suit different learning styles. Whether you prefer virtual learning or on-site training, our courses focus on practical skills that can be applied immediately.

If you’d like to build confidence working with REST APIs or explore where they fit within your development journey, contact the StayAhead team to discuss the right learning route for you.