Chris Nelson's Wiki

GameObject: Represents the type of objects which can exist in a Scene

MonoBehaviour: The base class from which every Unity script derives, by default.

Object: The base class for all objects that Unity can reference in the editor.

Transform: Provides you with a variety of ways to work with a GameObject’s position, rotation and scale via script, as well as its hierarchical relationship to parent and child GameObjects.

Vectors: Classes for expressing and manipulating 2D, 3D, and 4D points, lines and directions.

Quaternion: A class which represents an absolute or relative rotation, and provides methods for creating and manipulating them.

ScriptableObject: A data container that you can use to save large amounts of data.

Time (and framerate management): The Time class allows you to measure and control time, and manage the framerate of your project.

Mathf: A collection of common math functions, including trigonometric, logarithmic, and other functions commonly required in games and app development.

Random: Provides you with easy ways of generating various commonly required types of random values.

Debug: Allows you to visualise information in the Editor that may help you understand or investigate what is going on in your project while it is running.

Gizmos and Handles: allows you to draw lines and shapes in the Scene view and Game view, as well as interactive handles and controls.