Backspacing is another word for Offset... You can determine the offset of the wheel by measuring the backspacing... Its more commonly refered to when purchasing / finding the correct size as Offset...
0 offset is directly in the center of the wheel... +25 is 25mm closer to the face of the wheel...
here is an image courtesy of ENKEI Wheels of basic wheel definitions...
Enkei's definition of Offset...
OFFSET
The offset indicates the distance from the center of the rim to the attaching disk part (unit:m/m). As seen in the attached drawing, the offset is indicated by "+" when the fitting surface extends beyond the central line of the rim and "-" when it is falls within. Caution is required to prevent the tire from coming into contact with the fender of the vehicle or the brake caliper if the offsets are very different.
http://enkei.com/Wh.Definitions.html
But remember... Not all wheels with the correct offset for you will have enough Pad Height to clear various brake calipers... So Pad Height is something to look out for as well...