Unity provides three UI systems that you can use to create user interfaces (UI) for the Unity Editor and applications made in the Unity Editor:
This page provides an overview of each.
UI Toolkit) is the newest UI system in Unity. It is designed to optimize performance across platforms, and is based on standard web technologies. You can use UI Toolkit to create extensions for the Unity Editor, and to create runtime UI for games and applications (when you install the UI Toolkit package.
UI Toolkit includes:
Unity intends for UI Toolkit to become the recommended UI system for new UI development projects, but it is still missing some features found in Unity UI (uGUI) and IMGUI.
The Unity User Interface (Unity UI) package (also called uGUI) is an older,GameObject-based UI system that you can use to develop runtime UI for games and applications. In Unity UI, you use components and the Game view to arrange, position, and style the user interface. It supports advanced rendering and text features.
See the Unity UI package documentation for the manual and API reference.
Immediate Mode Graphical User Interface (IMGUI) is a code-driven UI Toolkit that uses the OnGUI function, and scripts that implement it, to draw and manage user interfaces. You can use IMGUI to create custom Inspectors for script components, extensions for the Unity Editor, and in-game debugging displays. It is not recommended for building runtime UI.
Unity intends for UI Toolkit to become the recommended UI system for new UI development projects, but it is still missing some features found in Unity UI (uGUI) and IMGUI. These older systems are better in certain use cases, and are required to support legacy projects.
Your choice of UI system for a given project depends on the kind of UI you plan to develop, and the features you need support for.
For a comparison of the available UI systems, see the Comparison of UI systems in Unity.