back spacing is the distance from the inside lip (backside) of the rim to the mounting surface. to much b/s tires will rub the frame when making sharp turns. to little b/s tires will stick out. look at the sticky at the top of this section will tell you what b/s you need for different size wheels.
twin-10 is right. backspacing is the measurement you want.
offset is the relationship of the mounting flange to the center cross section of the wheel. if it is dead center, it is zero offset. by most definitions, if the mounting flange is offset to the inside, that is negative offset, and makes the wheel stick out further. positive offset puts the mounting flange towards the outside, and tucks the wheel in under the chassis. this is how ASE defines it. some respected engineers define it the opposite way. both agree that the center is zero.
an 8" wide rim with zero offset should have 4" backspacing. backspacing is the only measurement you need. the rest is just semi-useless knowledge.