ABC1:

A, B and C are the highest three social grades (or classes) commonly used to put a person or a group of people in to a class. Publishing (and other media) companies tend to use this classification system to sort their audience in to it, allowing advertisers to better attract that audience. This is, in many ways, a fairly crude measurement, but is used because it is widely understood and makes it easy for the media to gather data. Because it's so widely available, it is effective at comparing different publishing companies. The A1 and B1 are the highest social grades, the equivelent to upper class (A1) and middle-class (B1) whereas C1 is the equivelent to the poor, or the lower class. I will take this in to account when planning which audience I will decide to target.