Soup is a broad term that describes a liquid-based food, and isn't wholly defined by its ingredients. Therefore, all bisques are soups, but not all soups are bisques.
It's the liquid nature of a dish that makes it soup — that, and the fact that it must be cooked. It can contain vegetables, protein, water, milk, beans, stock, broth, and more.
Traditionally, the main ingredient in a bisque was some type of seafood, like lobster or shrimp. However, the definition of what classifies as bisque has broadened over time.
Today, seafood is not necessarily an essential ingredient in bisque. A modern bisque can feature pureed vegetables and the base might include rice or other starches.