The BART rails have five endpoints. These are:
Colma,
Richmond,
Pittsburg/Bay Point,
Dublin/Pleasanton, and
Fremont. Work to extend the Colma line to
SFO airport is currently in progress at the time of writing, and occasionally there is talk about extending the Pleasanton/Dublin line to
Livermore. Until construction finishes, the BX
bus line can be taken from the
domestic flights terminal of SFO to the Colma BART station.
Of course, mass transit in the Bay Area is never quite that simple. To extend your range in San Francisco, you can pick up the MUNI rail, which makes stops all around the city, costs only a dollar, and allows you to grab transfers for the city’s busses under the same name. You can pick up the MUNI rail at these BART stations: Embarcadaro, Montgomery St, Powell St, Glen Park, Balboa Park, and the Civic Station.
Looking to go further, still? At the end of the N Judah line of the MUNI rail, is the Caltrain station. This will take you through Silicon Valley and by the Great America amusement park in Santa Clara. It also passes by the ACE Train rail, an express commuter train that runs from San Jose all the way to Stockton at a very infrequent schedule.