I turned my neck very fast and now it hurts. Is this what whiplash is?

Probably not. You have probably sustained a muscle strain as opposed to a true "whiplash" injury. Whiplash is a term for soft tissue, ligament, muscle, and joint injury from a rapid acceleration or deceleration action. Whiplash can include muscle strains, but the injury you describe is quite low energy, and therefor a simple muscle strain is most likely.