Density gives us a measure of how much mass is there in a given volume of a substance. It can also be defined as the ratio of mass and volume of a substance. Mathematically, we can write density as:
density = mass/volume
Since, mass is reported in units or grams (g), kilograms (kg), etc. and volume is measured in the units of liter (l), cm^3 or m^3; the units of density are g/cm^3, kg/l, kg/m^3, etc. Water has a density of about 1 kg/l or 1 g/cm^3 or 1000 kg/m^3.
A term commonly used with density, especially while working with fluids, is specific gravity. This is the ratio of density of a substance to that of a standard fluid (typically water). A specific gravity of more than 1 (Such as honey) suggests the material will be denser than water and hence will fall to the bottom of a water containing vessel. A specific gravity of less than 1 (say for petrol or gasoline), will mean the material will float over water.
Note that density is a function of temperature.
Hope this helps.