An API (Application Programming Interface) is a set of rules and protocols that allow different applications to communicate with each other. The most commonly used APIs are REST, SOAP, and GraphQL. Here are their main differences: 

• REST (Representational State Transfer): An architectural style using the HTTP protocol to interact with resources via URLs. It returns data in various formats (JSON, XML, etc.), with JSON being the most used for its lightness. REST is stateless, meaning each request must contain all the information needed for its processing. 

• SOAP (Simple Object Access Protocol): A standard protocol for exchanging XML messages. It is more complex than REST but offers robust security standards (WS-Security). SOAP can operate in a stateless or stateful manner, allowing session management. 

• GraphQL (Graph Query Language): Designed by Facebook, GraphQL allows clients to request exactly the data they need. Unlike REST, which relies on distinct resources, GraphQL uses a single endpoint to return the information specified by the client in JSON format. 

The choice of API type depends on the specific needs of the application. REST is often preferred for modern web applications, SOAP for environments requiring high security standards, and GraphQL for complex applications with precise data needs.