If you have no clue what an API is, make sure to check out our previous articles here and here. You don't want to miss these articles there before you continue reading here ;)

Internal / Private APIs

"Internal APIs," also known as "Private APIs," are disclosed by internal systems only. Those types of APIs are used in companies for their internal systems. They are "hidden" from external users since they are not listed in public APU directories and are not accessible from the outside world.

Only authorized personnel can access an API developer portal to change, remove, or add new internal APIs. They help increase efficiency in various areas such as product development, logistics, HR, and customer service.

Partner APIs

"Partner APIs" can be described as something "in-between Private and Public" APIs. Although they are listed in public API directories and therefore "publicly available," it's only to a limited extent. Also, you'll need special authorization as a requirement to gain full access most of the time. Once you've obtained access, you can communicate across company boundaries with other approved external systems. As a result, they are beneficial when working with strategic business partners. And to guarantee safety, Partner APIs have much higher security measures than Internal APIs. Those APIs are either free to use when working with ongoing business partners/partner companies or are sold as pay-by-use models. 

External APIs / Public APIs

"External APIs," also known as Public APIs, are open-source APIs that are publicly available to every software developer. Similar to Partner APIs, they are listed in public API directories but, in contrast, don't require special access. Public APIs can be freely exchanged, grant universal access, and give developers more flexibility than proprietary software and web services. Also, Public APIs are not restricted to a specific sector or industry, which offers clear advantages.

• It ensures flexibility to the company's infrastructure since it's independent of single propriety software and technology.
  
• The flexibility doesn't end with the infrastructure of the company. Public APIs also offer more options when it comes to customization of interactions and data collection. Therefore, companies can specially tailor their service and products to their customer's and user's needs.
  
• Sharing data primarily via APIs offers high availability and opens up new opportunities in both computing power and increases the gathered information's value.
  
• Open-Source APIs indeed are far more cost-effective for businesses and enterprises than proprietary software regarding upfront investment and ongoing costs.
  
• Most developer communities, especially around open-source projects, are excellent sources for expertise, tips, suggestions of application enhancements, and mental support ;)