The Silicon Valley lies at the southern tip of the San Francisco Bay. Once best known for its prune orchards, the area underwent explosive growth with the creation of the high-tech industry in the 1960s. Although much of the area suffers from the suburban sprawl typical of much of the western United States, it still has some remarkable charm. The nearby Santa Cruz mountains make for a welcome respite from bustling 21st-century cyberliving.
The term Silicon Valley was invented in the mid 1970s. Naturally, the local residents had names for their region prior to this newfangled name, such as "Santa Clara Valley" and "Valley of Heart's Delight," and still use them. The term Silicon Valley overlaps several of the pre-existing names for this region including parts of the South Bay and Peninsula.
Because the electronics industry is considered somewhat prestigious, nearby communities often redefine the term Silicon Valley to include themselves. Some of these communities were mostly farmland when the term was invented, so it was pretty natural that the term didn't originally include them, but they might reasonably be considered part of the Silicon Valley now. On the other hand, the Mercury News's Silicon Valley 100 Index extends the term to such fabulous lengths that even Watsonville — a small coastal community on the other side of the Santa Cruz mountains — is included.
So if you're looking to visit the Silicon Valley as a tourist, look to Palo Alto, Santa Clara, and the museums of San Jose. But if you're visiting a company which is "in the Silicon Valley," you may have to look farther afield around the South Bay, Peninsula, and East Bay regions.