API is a set of tools for building application software. An API may be used to develop web-based systems, operating systems, databases and more. API is used to simplify program development, as an programming interface of an application visually makes it easier to understand the rules of using certain technologies while building applications. API is usually related to some software library and describes and prescribes the behavior of certain tools, with the library itself being the actual implementation of the tools. APIs are commonly used in blockchain development and in the cryptocurrency industry. Blockchain API may be used to implement a payment method which accepts Bitcoin for your site/application or to build a new decentralized application on the peer-to-peer network. There are also APIs for creating non-financial protocol for the blockchains and to improve the speed of data flows within a company.