Silicon Valley is in northern California from San Francisco to San Jose, primarily on the western side of the Bay. The area is home to many high-tech companies, including Hewlett Packard, Intel, Adobe, Yahoo, Google, Apple, Twitter, Cisco, Oracle, and AMD.
Silicon Valley is also a two-tiered society of the haves and the have-nots. It has a large homeless population, and poor people living in rundown trailers camp out along one side of Stanford University, not far from the headquarters of Google. Silicon Valley companies rely extensively on work visas to exploit foreign labor, thereby denying engineering opportunities to Americans.
Silicon Valley is a valley in the geographic sense. The proper name for the valley in which much of Silicon "Valley" resides is the Santa Clara Valley.