Bolt pattern / lug pattern / bolt circle / pcd is defined by the number of bolt holes and the distance measured from the center of one lug hole to the farthest lug hole accross from it.  This is often measured in millimeters (25.4mm=1inch).

Offset  The distance from the centerline of the wheel to the mounting surface of the wheel measured in millimeters.

Hub Diameter / center bore (cb)  is the hole at the center of the wheel, this is often measured in millimeters.

Rear spacing / back spacing is the distance from the backside of the wheel mounting pad to the outside of the rim flange.

Negative offset:  When the back of the bolt pad is closer to the inside of the wheel; when mounting surface is inboard of the rim centerline.

Positive offset:  When the back of the bolt pad is closer to the street side of the wheel; when the mounting surface is outboard of the rim centerline.

Mounting Pad:  This is the area that will be used to contact the hub of the car.

Drop Center:  Not refered to in the image above, this is the area where the wheel dips down after the mounting hump or on a wheel with a deep lip, the area that dips down after the lip on the barrel.

It should be noted that a wheel that is 18x9 is measured from wheel bead to wheel bead, not wheel flange to wheel flange.  Typically a wheel that is 18x9 is 10 inches wide in total width.