API
Application Programming Interfaces are a set of definitions that describe how to use a piece of software. APIs translate requests submitted to a program so that it can then execute them internally. APIs may be accessed by users through their software clients or by other programs. Each of the programs in a microservice architecture communicate through APIs, either locally or across the internet.
In the context of distributed ledger technologies like blockchain, nodes expose APIs to allow outside parties to interact with the ledger. The API relays messages through a virtual machine, which contains the ABIs necessary to interact with its smart contracts.
REST API
REST (Representational State Transfer) is a design pattern for building distributed systems and APIs. It is based on the principles of statelessness, uniform interface, layered system, and cacheable responses. REST APIs provide a way for two systems to communicate over HTTP in a standard, structured format. REST APIs use HTTP methods such as GET, POST, PUT, and DELETE for operations on resources. These resources are exposed as endpoints, and the data is sent and received in formats such as JSON and XML.