Wheels: positive or negative offset

Could someone please explain positive and negative offset on wheels? Also how is offset measured?

I believe positive offset is when the center line of the wheel is on the outside of the mounting point.

according to tirerack.com 's wheel tech section you are right. they have nice diagrams, cross section of a wheel, dotted lines and such.
Also, look up a wheel shop in your area to see what kind of space you have to work with, and if they can change the off set of your wheels. I have a classic speedster body with IRS that has the brazil mangel chrome rims that have been modified to 7". The off set has needed to be changed due to adding rear disks. My 195/65 15 tires squeeze in there and sit fatter than with a 5.5" rim. A local shop that I've used does this and they can't be the only ones. (Cut out the center and weld them where ever to accomodate the space.) if you look closely at some pics I've posted you will be able to see the diff betweeen the front and rear wheels. Fronts look more deep dish.

Thanks Victor,

I went to Tirerack.com and was corrected in my belief. Positive offset is when the wheel center line is inside the mounting plane.