Cardboard is a paper-like material. It is strong and light weight by design because of two outer layers of paper glued together with a core of corrugated paper.  

Cardboard is commonly used as a packaging material, such as cardboard boxes. Cardboard is lightweight and strong, made from course grain paper (often recycled paper). Cardboard is strong as it has an inner core of rippled paper which is adhered between two sheets of flat paper.

Card Stock is another paper product, often mistakenly called card board, see: card stock similar products by name are Mat Board and Bristol Board.