This forum is for UI Toolkit (formally named UIElements), which was introduced in Unity 2019.1 as new UI API that is currently only available for the Editor.
Docs: Unity - Manual: UI Toolkit
Namespaces: UnityEngine.UIElements
and UnityEditor.UIElements
For uGUI, or Unity UI - current way to create in-game UI:
Forum: Unity Engine - Unity Discussions
Docs: Redirecting to latest version of com.unity.ugui
Namespaces: UnityEngine.UI
and UnityEditor.UI
For IMGUI, or OnGUI() - pre-2019.1 way to create Editor Extensions:
Forum: Unity Engine - Unity Discussions
Docs: Unity - Manual: Immediate Mode GUI (IMGUI)
Namespaces: UnityEngine
and UnityEditor