Define density. How is it related to floating and sinking in water?

Density- 
Density of a substance is its mass per unit volume. It varies with temperature and pressure.

$\mathrm{Density} = \frac{\mathrm{Mass}}{\mathrm{Volume}}$

It is related to sinking and floating in water. If a substance is denser than water it will sink in water and settles down at bottom. If a substance floats on water it is less denser than water. For example, oil droplets floats on water surface while sand particles settle down at bottom.