Density is mass per volume. Physics and engineering tables use kilograms per cubic meter, chemistry and materials data use grams per cubic centimeter (numerically the same as kilograms per liter), and US construction and shipping use pounds per cubic foot. Water near 4 °C is almost exactly 1,000 kg/m³, which makes it a handy reference.
Use the converter above for any pair. The most searched pair, kg/m³ to g/cm³, has its own page with a table of common materials.