Offset is the distance from the centre line of the wheel, to the mounting face.

You get Negative offset, and positive offset.

Negative offset means the mounting face is closer to the outside of the wheel, positive is the inside. ET0, a zero offset, means the mounting face is exactly in the centre.

It has nothing to do with the width of the rim. A great site for working out offsets and widths is http://www.willtheyfit.com/