With their definitions, the differences between density and relative density can be established. Density refers to a measure of the mass of matter contained by a unit volume. The density of an object is proportional to its mass. This means you might have two substances of the same volume, but different densities.
For instance, if substance A has more mass than substance B, substance A will have more density than substance B, even if they are of the same volume. The reason is that an object with a greater mass will always have a higher density.
However, if two substances have the same mass but different volumes, the one with lower volume will have higher density. On the other hand, relative density is a measure of the density of a material by calculating the ratio of the density of a substance to the density of water at the same temperature. Unlike density, relative density does not have any unit.