Quaternions represent rotations and orientations.
Quaternions are four dimensional, but don't be frightened—unless intending to learn their mathematical implications—this doesn't affect practical usage in Unity.
To use them you don't need to understand how they work, or what their components represent.
To explore outside the scope of this document, look to the resources in the sidebar.
Quaternions are not Euler angles.
What this means:
Unity, huh, how?
x, y, z.x, y, z, and w are for advanced use cases only.Just as a Vector3 can represent a position or a direction, a quaternion can represent either an orientation or a rotation.
An orientation is a rotational placement, similar to position. transform.rotation is the global orientation of a transform.
A rotation is a manipulation of another orientation or rotation, similar to a movement vector.
Sadly, rotation is a general term and is often used to describe both orientations and rotations. In some contexts there's no reason to distinguish one from another, but you may find this helpful.