Memory foam is a type of pillow which is composed of viscoelastic, or "memory", material. The principle behind memory foam is that it absorbs and evenly distributes body weight and pressure so your neck, head and back can breathe easily, leading to better spinal alignment. Memory foam pillows are popular because they conform to the shape of the human head for a more comfortable sleep. Learn more about "Which Type of Memory Foam Pillow is Best?"