Measuring the wheel's diameter is done by simply measuring across its center, and the wheel width is measured across the barrel from front to back. The bolt pattern is the number and arrangement of the lug nuts that hold the tires on.
To measure the offset of a wheel, measure the distance between the hub mounting surface to the center line of the rim. Putting the wrong size wheels and tires on a vehicle can greatly affect the vehicle's performance and handling. Wider tires do not help a car's ability to grip the road and can result in a decrease in cornering ability and a decrease in the vehicle's ride quality.