Architecture of the Neck
The cervical spine can be easily injured from a whiplash in an accident or stiff and misaligned from poor postures while working at a desk for long periods of time.
There are discs that cushion the vertrabrae. Whenever you exercise the neck, move slowly without throwing the head around on the neck. The neck is a pivot joint and should be exercised minimally and carefully to prevent injury.
Appropriate Neck Exercises
Stiffness is usually the result of keeping the head in one position and the supporting muscles around it tire, such as SCM (sternoclastomastoid) that attaches the head to the neck at the front of the chest.
Exercises such as found in yoga including headstand (Sirsasana) is not necessarily a good idea for most people. The neck muscles tend to not be the strongest muscles in people and the upper back, chest and belly muscles should do the work of keeping the neck safe in a headstand. This is an example of a very common exercise that can injure the neck: the entire weight of the body is balanced on top of the head and moves through the neck bones. Instead, work slowly with a skilled teacher or do a much healthier form of neck traction.