What is the difference between density and specific gravity?

Measuring or comparing

Density and specific gravity are both used to describe mass and can be used to compare substances. However, they are not identical measures. 

• Density is defined as mass per unit volume. It has the SI unit kg m^-3 or Kg/m^-3 and is an absolute quantity.
• Specific gravity is the ratio of a material's density with that of water at 4 °C (where it is most dense and is taken to have the value 999.974 kg·m^-3).  It is therefore a relative quantity with no units.

The words ‘taken to have the value’ are important here, as the value of maximum density cannot be known exactly (nor, for that matter, can the temperature of 4 °C be realised exactly).