What is RESTful API?
RESTful API is an interface that two computer systems use to exchange information securely over the internet. Most business applications have to communicate with other internal and third-party applications to perform various tasks. For example, to generate monthly payslips, your internal accounts system has to share data with your customer's banking system to automate invoicing and communicate with an internal timesheet application. RESTful APIs support this information exchange because they follow secure, reliable, and efficient software communication standards.

What is an API?
An application programming interface (API) defines the rules that you must follow to communicate with other software systems. Developers expose or create APIs so that other applications can communicate with their applications programmatically. For example, the timesheet application exposes an API that asks for an employee's full name and a range of dates. When it receives this information, it internally processes the employee's timesheet and returns the number of hours worked in that date range.

You can think of a web API as a gateway between clients and resources on the web.

Clients
Clients are users who want to access information from the web. The client can be a person or a software system that uses the API. For example, developers can write programs that access weather data from a weather system. Or you can access the same data from your browser when you visit the weather website directly.

Resources
Resources are the information that different applications provide to their clients. Resources can be images, videos, text, numbers, or any type of data. The machine that gives the resource to the client is also called the server. Organizations use APIs to share resources and provide web services while maintaining security, control, and authentication. In addition, APIs help them to determine which clients get access to specific internal resources.

What is REST?
Representational State Transfer (REST) is a software architecture that imposes conditions on how an API should work. REST was initially created as a guideline to manage communication on a complex network like the internet. You can use REST-based architecture to support high-performing and reliable communication at scale. You can easily implement and modify it, bringing visibility and cross-platform portability to any API system.

API developers can design APIs using several different architectures. APIs that follow the REST architectural style are called REST APIs. Web services that implement REST architecture are called RESTful web services. The term RESTful API generally refers to RESTful web APIs. However, you can use the terms REST API and RESTful API interchangeably.

The following are some of the principles of the REST architectural style:

Uniform interface
The uniform interface is fundamental to the design of any RESTful webservice. It indicates that the server transfers information in a standard format. The formatted resource is called a representation in REST. This format can be different from the internal representation of the resource on the server application. For example, the server can store data as text but send it in an HTML representation format.

Uniform interface imposes four architectural constraints:

Requests should identify resources. They do so by using a uniform resource identifier.
Clients have enough information in the resource representation to modify or delete the resource if they want to. The server meets this condition by sending metadata that describes the resource further.
Clients receive information about how to process the representation further. The server achieves this by sending self-descriptive messages that contain metadata about how the client can best use them.
Clients receive information about all other related resources they need to complete a task. The server achieves this by sending hyperlinks in the representation so that clients can dynamically discover more resources.
Statelessness
In REST architecture, statelessness refers to a communication method in which the server completes every client request independently of all previous requests. Clients can request resources in any order, and every request is stateless or isolated from other requests. This REST API design constraint implies that the server can completely understand and fulfill the request every time. 

Layered system
In a layered system architecture, the client can connect to other authorized intermediaries between the client and server, and it will still receive responses from the server. Servers can also pass on requests to other servers. You can design your RESTful web service to run on several servers with multiple layers such as security, application, and business logic, working together to fulfill client requests. These layers remain invisible to the client.

Cacheability
RESTful web services support caching, which is the process of storing some responses on the client or on an intermediary to improve server response time. For example, suppose that you visit a website that has common header and footer images on every page. Every time you visit a new website page, the server must resend the same images. To avoid this, the client caches or stores these images after the first response and then uses the images directly from the cache. RESTful web services control caching by using API responses that define themselves as cacheable or noncacheable.

Code on demand
In REST architectural style, servers can temporarily extend or customize client functionality by transferring software programming code to the client. For example, when you fill a registration form on any website, your browser immediately highlights any mistakes you make, such as incorrect phone numbers. It can do this because of the code sent by the server.