What is a user-friendly interface to a service's APIs known as?

A user-friendly interface to a service's APIs is often referred to as a Graphical User Interface (GUI). This is because a GUI provides visual elements like buttons, icons, and menus that enable users to interact with software applications in an intuitive way. It simplifies the complexity of APIs, allowing users to perform functions and access features without needing to write code or understand the underlying technical details.

By offering a visual representation, GUIs are designed to enhance user experience, making it easier to navigate and utilize services. This contrasts with other options: an SDK (Software Development Kit) typically consists of tools and libraries for developers to build applications, an API gateway serves as an intermediary that manages and routes API requests, and a web interface is a broader term that could encompass GUIs but doesn't necessarily imply a user-friendly design focused on ease of use.

Thus, the concept of a GUI specifically addresses the intent of providing a more accessible and user-friendly way to interact with APIs, firmly establishing it as the correct answer.