Since earthquakes are produced by sudden slip along faults, it follows that understanding the nature of faults is a key to understanding the nature of earthquakes. A fault, in the geologic sense, is a roughly planar fracture in the Earth's crust along which slip -- the relative offset of the two sides -- has occurred. Faults can be active, meaning that they currently hold the potential for producing earthquakes, or inactive, meaning that although they once slipped and produced earthquakes, they are now "frozen" solid. If the tectonic environment of an area changes, however, inactive faults can sometimes be reactivated.
In terms of size, faults can be anywhere from less than a meter to over a thousand kilometers in length, with a width of a similar scale. The depth of very large faults is constrained by the thickness of that portion of the Earth's crust and lithosphere in which brittle fracture can occur. In southern California, this depth is roughly 15 to 25 kilometers. The kind of faults seismologists study are generally at least a square kilometer in area, and typically more than a hundred square kilometers in area. Faults of this size or greater can "break", or rupture, violently enough to produce significant earthquakes. There are approximately 200 faults in southern California that are considered major faults, capable of producing damaging earthquakes. Smaller faults -- of which there are countless thousands, even millions, in southern California -- will only produce very minor tremors. Large faults can also produce minor earthquakes, if they rupture only in part, and not along their entire length.
This is one of the most basic connections between faults and the earthquakes they generate. With few exceptions, the size of the fault rupture area is directly proportional to the size of the earthquake produced by the slip along that area. In other words, the greater the fault area that slips, the greater the earthquake produced. Keep in mind, though, that the actual rupture area of an earthquake is not always equal to the total surface area of the fault that ruptures -- often, only a small fraction of that total (potential) area actually slips.