To prevent people's heads from being snapped back, causing neck injuries in rear-end crashes, head restraints are required in the front seats of all new passenger vehicles. Rear-seat head restraints aren't required but are found in more and more cars.
All head restraints aren't the same. Some are adjustable, and some are fixed. Head restraints also vary a lot in height and how far they're set back from occupants' head. To prevent neck injuries, it's necessary for head restraints to be directly behind and close to the backs of occupants' heads. Make sure the ones in a car you're considering can be positioned this way. In general, fixed head restraints are preferred because they don't have to be adjusted for different occupants. If the ones in a car you're considering are adjustable, check that they can be positioned behind and close to the back of your head. Make sure they lock when adjusted, because some don't.
Don't be surprised if a head restraint cannot be positioned for adequate protection. Among more than 200 1997 model passenger cars in which the NHTSA measured the head restraints, more than half had poor geometry. Only five vehicles had good head restraint geometry. They were the Honda Civic del Sol, Mercedes E class with restraints that adjust automatically, the Toyota Supra, and two Volvo models -- the 850 and 960.