38mm offset is more inward than a 30mm offset rim, and is pretty close to stock. The lower the number the more they are pushed out past the fender. 35mm is flush with the fender. 30mm puts the wheel outside of the fender. These numbers are estimates, as camber, and wheel and tire sizes will affect the position relative to the fender.
Wheel offset is measured from the centre of the wheel rim to the wheel mounting flange. Offset can be either positive or negative depending whether the face of the wheel mounting-flange is to the outside (positive) or to the inside of the wheel (negative). For example, an offset of zero would mean the face of the wheel mounting-flange would be aligned with the exact centre of the wheel rim.
Tip: The offset is usually printed or stamped on the back of the rim.
See also: http://www.yokohamatire.com/utcustom.asp