A crack in the Earth's crust, almost 1000 km long, running from the San Bernardino area north past San Francisco. The famous San Francisco fire of 1906 was caused by a slippage of this fault line, along the SAFZ (San Andreas fault zone).
The San Adreas is a strike-slip fault, meaning the two sides move in opposition - see liha's writeup below for the mechanics. In the great 1906 earthquake the displacement along the fault was 21 feet near Point Reyes.
The fault's major stress point is thought to be Black Mountain near Palo Alto, where the fault line changes direction by about 10 degrees.