Density is a property that describes how much mass is contained within a certain volume, and is calculated by dividing an object's mass by its volume. It varies between substances, with water being denser than oil, Styrofoam less dense than ceramic, wood floating but rocks sinking due to their higher density than water, and helium balloons rising because helium is less dense than air. Knowing an object or substance's density can help predict if it will float or sink.