The transform fault that separates the Pacific and North American plates in California is the San Andreas Fault. It is a major right-lateral strike-slip fault, meaning the two sides move mostly horizontally past one another rather than one plate sinking beneath the other.
The fault extends roughly 750 miles through California and continues toward the Gulf of California. The Pacific Plate moves generally northwest relative to the North American Plate, producing frequent earthquakes as sections of the fault lock, accumulate stress, and then slip.
The San Andreas is a fault system with many related branches, including the Hayward and San Jacinto faults. Those nearby faults are important earthquake sources, but they are not the canonical answer to this question. The 1906 San Francisco earthquake and the 1989 Loma Prieta earthquake were associated with the San Andreas system, although the 1989 rupture occurred on a nearby part of the system.