REST APIs (Representational State Transfer Application Programming Interfaces) and Microservices are both crucial concepts in the development of modern web applications, but they refer to different aspects of software design and architecture. Understanding their key differences is essential for effectively employing them in building scalable and maintainable applications.

REST APIs

1. Definition: REST APIs are a set of principles and constraints for building web services that allow clients (such as web browsers, mobile apps, or other services) to interact with a server's resources. REST is an architectural style that uses HTTP requests to access and use data. The data that an API returns can be in various formats, with JSON (JavaScript Object Notation) being the most popular.

2. Focus: The focus is on the communication between the client and server, specifically on how resources (data or functionality) are accessed and manipulated over the web.

3. Statelessness: One of the core principles of REST is that it is stateless, meaning the server does not need to know anything about the client's state and vice versa. Each request from the client to the server must contain all the information needed to understand and complete the request.

4. Use Cases: REST APIs are used for web services that need to be accessible and manipulable through the internet, including social media platforms, e-commerce sites, and cloud services.

Microservices

1. Definition: Microservices is an architectural style that structures an application as a collection of small, autonomous services modeled around a business domain. Each microservice is self-contained and implements a specific business functionality, runs in its own process, and communicates with other services via well-defined interfaces.

2. Focus: The focus is on decomposing an application into smaller, independent components to improve modularity, scalability, and maintainability. This approach allows teams to develop, deploy, and scale parts of an application independently.

3. Autonomy: Each microservice is developed, deployed, and managed independently. It can be written in different programming languages and use different data storage technologies. The microservices communicate with each other using lightweight protocols, often HTTP/REST, but also others like gRPC.

4. Use Cases: Microservices architecture is suited for complex applications that need to be efficiently scaled and rapidly evolved. Examples include e-commerce platforms, streaming services, and enterprise applications that require high agility and scalability.

Key Differences

• Scope and Concept: REST API is a set of guidelines for building stateless web services using HTTP. Microservices is an architectural style for structuring applications as collections of independently deployable services.
• Granularity: REST APIs are concerned with the interface for accessing individual resources or operations over the web. Microservices architecture deals with the larger structure of the application, defining how the application's distinct features and functionalities are separated into independently serviceable components.
• Application Structure vs. Communication Protocol: Microservices architecture defines the structure of an application as a set of loosely coupled services. REST APIs are a way for these services (and external clients) to communicate over the web, but the communication could also be achieved using other protocols or styles.
• Independence: Microservices emphasize operational independence and autonomy, where services can be developed, deployed, and scaled independently. REST APIs focus on the stateless exchange of information and can be used within a microservices architecture or in a monolithic application.

In summary, while REST APIs are a convention for structuring interactions between clients and servers, microservices architecture is a strategy for structuring the application itself. They are often used together, with microservices communicating internally and externally through RESTful interfaces, but they address different aspects of software design and development.