Density is a measure of how heavy something is for its size. It is defined as mass per unit volume. The formula for calculating density is mass divided by volume (D=m/v). Units for density are either grams per milliliter (g/mL) or grams per cubic centimeter (g/cm3). Since 1 mL = 1 cm3, it does not matter what unit you use. Usually mL measures liquid volume while cm3 measures solid volume.
Source: Jennifer Roushar
Different substances have different densities as shown in the table below. The larger the density the heavier the object is for its size. Since the density of water is 1.000, anything with a density of greater than 1 will sink in water and anything with a density of less than 1.000 will float in water.
Source: Jennifer Roushar; Image from wiki.chemeddl.org
Source: Jennifer Roushar
Source: Jennifer Roushar