Here is a typical wheel size: 17x7 5x100 50s. Below is an example of the wheel size broken down so you can better understand each element.
Wheel Diameter: The size of the wheel as measured across it's center from the outside. Remove hubcap before measuring.
Wheel Width: The width of the wheel measured in inches. It's the measure across its barrel from the front rim to the rear rim.
Bolt Pattern: The bolt pattern indicates the layout of lug holes and consists of two elements: the number of bolts, or lugs (5 in this case), and the space between them across the center of the wheel (100mm in our example).
Offset: Offset is the distance from a wheel's mounting surface on the hub to the centerline of the wheel, in millimeters. Negative offset means the mounting surface is towards the inside of the wheel. With positive offset, this surface is biased towards the outside. Typically, wider tires require more positive offset to avoid contact on the inner fender well.
Finish: There are many different wheel finishes on a variety of wheels. They are clearly marked at the end of the wheel size (ex. S = Silver).