BART (Bay Area Rapid Transit)
BART is a regional commuter rail that connects the northern parts of the Peninsula to San Francisco, the East Bay, and both SFO and Oakland International Airports. Most of the Peninsula and South Bay can connect to BART at the Milbrae Station via Caltrain.
For those commuting from Silicon Valley and working in San Francisco, BART and Muni (the City's bus and light rail system) are the main methods of transit once you are in the City. Below are some methods of getting into the City from the Peninsula and South Bay.
Caltrain is a commuter rail connecting San Francisco to the Peninsula and South Bay, ultimately reaching Gilroy. As the Peninsula and South Bay are lesser densely populated areas than San Francisco, Caltrains departures come hourly, with more frequent service at peak travel times.
SamTrans (San Mateo County Transit)
SamTrans is a bus system for San Mateo County, connecting into portions of San Francisco and Palo Alto. Largely concentrated east of the Santa Cruz Mountains, it services many communities within the Peninsula. SamTrans is considered an adjunct service to Caltrain and VTA, and the monthly passes for these services also include SamTrans fares.
VTA (Santa Clara Valley Transportation Authority)
VTA is a network of buses and light rail servicing Santa Clara County, which includes several communities that make up the Silicon Valley, or the South Bay. Many VTA lines connect to Caltrain or even BART.