Segmentation is the breaking up of a fault along its length into several smaller faults. This can happen as a result of other faults crossing it, topography changes, or bends in the strike of the faults. Segmentation can limit the length of faulting in a single earthquake to some fraction of the total fault length, thus also limiting the size of the earthquake.
The segments of the San Andreas Fault, and the probability for strong shaking in the next 30 years on each segment. (Image from Working Group on California Earthquake Probabilities, 1988)