Rigidbody.rotation allows you to get and set the rotation of a Rigidbody using the physics engine. If you change the rotation of a Rigidbody using Rigidbody.rotation, the transform will be updated after the next physics simulation step. This is faster than updating the rotation using Transform.rotation, as the latter will cause all attached Colliders to recalculate their rotation relative to the Rigidbody.
If you want to continuously rotate a rigidbody use MoveRotation instead, which takes interpolation into account.
Note: rotation is world-space.