Each wheel has a different bolt pattern, and some wheels even have 2 different bolt patterns which allow it to be mounted on a wider range of vehicles.
Most Bolt Patterns are represented in the following manner:
The "4" indicates the number of holes in the wheel for the bolts to enter and mount the wheel onto the car.
The "100" indicates the diameter of the bolt circle measured in millimeters or inches. 4 & 6 bolt wheels are measured from the center of one bolt hole to the center of the bolt hole directly across from it. On a 5 bolt pattern, it is a bit trickier to measure without special tools. Imagine a circle running through the centers of each bolt hole. You would measure from the center of one bolt hole to the imaginary circle that lays between the opposite two bolt holes.