Faults slice through the earth’s crust and allow large pieces of the crust, called tectonic plates, to slide past each other.
As shown in this diagram of the earth created by NASA, the crust is actually much thinner relative to the earth than an orange peel is to an orange!
The map below shows the dozen or so major plates that make up the crust. The San Andreas fault (and the other smaller faults in the Bay Area) mark the boundary between two of these plates.
Find out more information about plate tectonics
Source: NASA
Source: USGS