How does a perspective drawing differ from an isometric drawing of the same object

Appreciate your help and thank you in advance :)

An isometric drawing is drawn to scale. It gives a 3d representation of an object.
A perspective drawing is not drawn to scale, it is drawn to "vanishing points" and gives a more realistic view of the object. Everything we look at is seen by our eyes in a perspective view.

Isometric is a term used for math and engineering that is used to visualize a 3D shape. An edge is pointing out towards you and the angles of the other edges are at 120 degrees from that vertical line.

Here's a picture:
http://upload.wikimedia.org/wikipedia/commons/thumb/6/67/3D_shapes_in_isometric_projection.svg/220px-3D_shapes_in_isometric_projection.svg.png

Perspective is for art and has a horizon line down the middle. Usually there are vanishing points on that line so above the horizon line goes to the vanishing point and the below goes up to the vanishing point.

Isometric is looked at from above and doesn't have the horizon line.