what is the difference between specific gravity and density, please explain with some example?

Density is mass/volume so its' units are grams/cm^3

Specific Gravity is density compared to the density of another substance, usually water.

Density has units. Specific Gravity is unit less.

They can be numerically equal.

Consider the following:

Water has a DENSITY of 1 g/cm^2; Mercury has a density of 13 g/cm^3, thus Mercury is 13 times more dense than water or its SG is 13.

Density is defined as amount of matter per unit volume. This means in a given space , how much mass or simply a matter occupy space. Specific Gravity is a dimensionless quantity. It is the ratio of density of the substance to the density of reference ( water ,air,petrol.. depends on our choice) substance.