Rigidbodies are, along with Colliders, the core of the Physics engine beneath Unity. When they are attached to a game object, they place it under the control of the Physics engine, which will take care to properly move its Transform. In fact, they should be moved with other functions, and scripts shouldn't touch the Transform. We will see this in detail later.
The exact name of the component is Rigidbody 2D (whereas Rigidbody is for the 3D Physics engine, but for brevity many times we will use the term rigidbody to specify a Rigidbody 2D component).
From Unity 5.4, and then again in Unity 5.5, the Rigidbody 2D component has slightly changed. In fact, many improvements have been made to the Physics engine of Unity.
Once the component ...