What is an API? 

An Application Programming Interface (API), is a set of definitions and protocols for building and integrating application software. They allow disparate products or services to communicate with one another. This, in turn, will enable organizations to open application data and functionality to third parties, simplify application design and management, allow flexible development, and provide opportunities for innovation. 

Some of the best-known APIs include: 

  • “Sign in with . . .” technology
  • Price comparison sites
  • Google maps

Types of API

There are four main types of API: 

Open APIs

Open APIs, sometimes called external APIs, are shared freely on the internet, allowing outside developers or businesses to access them. An organization would typically publish an open API to encourage third-party developers to discover new ways to use their software. 

Public APIs

Like open APIs, organizations share public APIs online for third parties to use and experiment with. However, public APIs typically have more restrictions; some businesses will implement moderate authentication and authorization, and others will even monetize the API by imposing a per-call cost. 

Partner APIs

Used to aid communication between business partners, partner APIs are one of the most common forms of API. They generally have more stringent rules surrounding authorization and authentication than public or open APIs, especially if used to transfer sensitive information. 

Some of the best-known examples of partner APIs are:

Internal APIs

Most organizations use internal APIs exclusively for communicating between internal business applications; for example, between payroll and HR systems. They are not publicly available and, thus, typically have little to no authorization or authentication requirements. However, as threat awareness improves, organizations are beginning to change how they think about internal APIs and are implementing more stringent security measures. 

Composite APIs

By combining multiple APIs, composite APIs craft a sequence of related or interdependent operations, typically to address complex API behaviors or improve the speed and performance of individual APIs. 

API Formats 

Aside from types of API, there are also three main API formats, also known as protocols or architectures. They are: 


A Representational State Transfer (REST) API, or RESTful API, conforms to the design principles of the REST architecture. They provide relative flexibility and freedom for developers and are typically used to connect components and applications in microservices environments and exchanging data or resources, such as documents. 


Simple Object Access Protocol (SOAP) is a clear, tightly structured, controlled, and defined messaging standard for web APIs. SOAP supports a range of communication protocols, including HTTP, SMTP, and TCP/IP, and is style-independent and extensible, allowing developers to develop SOAP APIs in varied ways, easily adding features and functionality. 


Remote Procedural Call (RPC) is the most straightforward API format. Organizations primarily use it to send multiple parameters and receive results, invoking executable actions or processes. 

The Future of APIs

API usage has expanded in recent years, and this growth is only likely to continue. Unfortunately, just as the number of APIs has increased, so has the number of attacks launched upon them – and API security is only just beginning to catch up. We’ll likely see API security mature in the coming years, further enabling growth and innovation. 

For more essential cybersecurity definitions, check out our other blogs below: 

21 Essential Cybersecurity Terms You Should Know

40+ Cybersecurity Acronyms & Definitions

Return to Cybersecurity Glossary

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