Cervical pillows, despite their name, have very little to do with the anatomy of the lower body. Rather, in this case, "cervical" refers to the vertebrae directly below the skull - basically, the bones of the spine that form your neck. A cervical neck pillow is used for people who either have injury in that area or otherwise need to support and comfort their neck.
The same kinds of neck pillows are available at airports and rest stops, for providing comfort during long trips when a person will need to remain upright. These kinds of cervical pillows are also useful for easing snoring/sleep apnea, heartburn and acid reflux. It's best to listen to your doctor or care provider to determine which cervical pillow you should use. You will want to find one that fits your body - one that works best at keeping your neck in a neutral position or a straight line. Using a cervical pillow that doesn't sit in the right spot, or one that doesn't conform to your body, can cause further strain and end up doing more harm than good.
Some good features to look for in a quality cervical neck pillow on BudgetMedicalSupplies.com are pillows made of memory foam - a material that can mold itself to your body while you rest on it, but spring back to its original shape when you get up. Inflatable models that are durable and of good quality are also useful, and are easy to clean. There are cervical pillows that can be filled with warm or cool water for cushioning that conforms to your head and neck.