A Colloid is a solution that contains a substance which is uniformly distributed in a liquid.  Milk is an example of a colloidal solution.  Other examples are jelly, plaster, muddy water, etc.

A colloidal solution has two main components. They are the colloidal particles and the dispersing medium.  A key feature, which distinguishes colloids from suspensions is that a colloidal solution never settles.  The particles will always remain suspended. 

Colloidal solution exhibits a property called the Brownian movement.  The particles have a random, zig zag motion, when observed under the microscope.  This is due to collisions between the particles in the dispersing medium. 

When a beam of light is shone on to a colloidal solution, the particles on the path of the light will be illuminated.  For examples, on a foggy night, the path of light from a car head light can be seen.