Sometimes the term orthopedic pillow or SlumberPM Review Review chiropractic pillow is used as well. That being said, all Neck Pillows are built to give you support to keep your spine in alignment. Some may be shaped like a regular pillow (rectangle), some are square, or perhaps oddly shaped if you will. The shape or design of the pillow shouldn't make a huge difference; it may be more of a personal preference. All neck pillows should have some kind of neck support, meaning they will hold your spine in a neutral position. It should allow for a curvature in the neck when back sleeping and keep your spine straight when on your side.
They may often have different designs too. Maybe a wave-like design, or it can have a pocket where your head sits. Material is also another variable. I've seen, experimented with, and have reviewed pillows of all different materials. Fiber filled pillows, water based pillows, foam pillows, buckwheat pillows, etc... I tend to like fiber filled pillows that are standard firmness, not too hard and not too soft. This allows for good support and a comfortable night's sleep. Also, synthetic fibers are often hypoallergenic and don't hold dust mites like a feather pillow may.
I tend to find memory foam to be slightly too hard for many, and I don't want to play around and have to adjust my pillow like you sometimes have to with a water based pillow. But again, you may need to try some different options to see what you like best. Before choosing your neck pillow you often want to research what that particular pillow helps with the most. Some can be designed more for relieving specific conditions like headaches or snoring, some may be more for muscle tension relief, some may help with all of the above.