GameObjects are the fundamental objects in Unity that represent characters, props and scenery. They do not accomplish much in themselves but they act as containers for Components, which implement the real functionality. For example, a Light object is created by attaching a Light component to a GameObject.
The Transform component determines the Position, Rotation, and Scale of each object in the scene. Every GameObject has a Transform.
Rigidbodies enable your GameObjects to act under the control of physics. The Rigidbody can receive forces and torque to make your objects move in a realistic way. Any GameObject must contain a Rigidbody to be influenced by gravity, act under added forces via scripting, or interact with other objects through the NVIDIA PhysX physics engine.
This structure is used throughout Unity to pass 3D positions and directions around. It also contains functions for doing common vector operations.
The Navigation System allows you to create characters which can navigate the game world. It gives your characters the ability to understand that they need to take stairs to reach second floor, or to jump to get over a ditch.