noob question..wut is offset?

[Myteg]

i have always heard of offset wut i never understood. can someone explain??

 

[ixcocoyxi]

16x8 +0

16 is the wheel size
8 is the width
0 is the offset (where the center of the rim/spokes sits in relation to the outside of the lip)

a +40 would have the rim sit inside the wheel well and the face would have no/little lip
a +15 would have the rim sit out further, closer to the fender and the lip would be bigger.

 

[alyssafacex]

this is offset

 

[dc2GS-R]

Offset
The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel. The offset can be one of three types (measured in millimeters).*

Zero Offset
The hub mounting surface is even with the centerline of the wheel.

Positive
The hub mounting surface is toward the front or wheel side of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.

Negative
The hub mounting surface is toward the back or brake side of the wheels centerline. "Deep dish" wheels are typically a negative offset.

If the offset of the wheel is not correct for the car, the handling can be adversely affected. When the width of the wheel changes, the offset also changes numerically. If the offset were to stay the same while you added width, the additional width would be split evenly between the inside and outside. For most cars, this won't work correctly. We have test fitted thousands of different vehicles for proper fitment. Our extensive database allows our sales staff to offer you the perfect fit for your vehicle.

*Backspacing, similar to offset, is the distance from the hub mounting surface to the inside lip of the wheel (measured in inches).

Source:
http://www.tirerack.com/wheels/tech/techpage.jsp?techid=101