Web APIs function by delineating the endpoints (URLs) and defining how clients can request information or perform other actions by communicating HTTP queries (such as GET, POST or DELETE) to these endpoints. The API server processes the requests and returns an answer, usually in the form of JSON as well as XML.

There are several types of APIs:

RESTful APIs: Based on representational state transfer principles and use HTTP requests.

SOAP APIs: Use XML messages for communication and often have stricter rules.

Graph QL APIs: Allow clients to request only the specific data they need.

RPC APIs: Use remote procedure calls to invoke functions across systems.

An API is an arrangement of rules that permits different software applications to communicate with one another. Web services are an API type that can be accessed via networks, usually via HTTP. Therefore, a Web service is a type of API, however not all APIs are web services.

To access to use a Web API, you typically require an API key, or token issued from the provider. You can make HTTP requests to API's endpoints by using methods such as POST, GET or DELETE. You then process the responses in accordance with the API documentation.

API documentation contains information about how to utilize the API. It provides information on possible endpoints, requests and response formats, ways to authenticate, errors handling and examples that will help developers incorporate an API within their application.

API Versioning is the method to manage different variants of an API. As APIs develop there are changes which could cause disruption to the functionality of client applications. Versioning lets developers control and control these changes without affecting the existing users.

Rate limitation is a method employed in API providers to limit the number of requests that the client is able to make within a specific time period. It can help prevent the misuse of APIs, to ensure fair use and ensure the performance and stability of an API server.