The ones normal humans actually need to distinguish

San Fernando Valley — “The Valley.”

North of the LA Basin, over the Santa Monica Mountains. Sherman Oaks, Encino, Van Nuys, Northridge, Woodland Hills, Reseda, etc. This is the Valley of Valley Girl, Fast Times, strip malls, Ventura Boulevard, 105°F afternoons and somebody's dad owning a pool-supply company. Sherman Oaks Galleria was explicitly associated with the Valley Girl phenomenon in the 1980s.

San Gabriel Valley — absolutely not The Valley.

East/northeast of downtown LA. Pasadena, Alhambra, San Gabriel, Arcadia, Monterey Park, etc. Enormous Asian-American cultural presence, excellent food, entirely different LA subcivilization. Calling someone from Monterey Park a “Valley Girl” because the words San Gabriel Valley appeared on Google Maps would be an impressive failure of California studies.

Santa Clarita Valley — also not The Valley.

Immediately north of the San Fernando Valley. Valencia, Newhall, Canyon Country. Master-planned subdivisions, Six Flags, commuting. It is a valley next to The Valley, thereby creating the kind of nomenclature California enjoys inflicting upon newcomers.

Antelope Valley — definitely not The Valley.

Lancaster/Palmdale and the high desert. Aerospace, wind, enormous sky, Joshua trees. Technically much of it is still LA County, which is why “I live in Los Angeles County” can mean either Beverly Hills or an F-35 flying over your backyard.

Santa Clara Valley — the physical basis of Silicon Valley.

San Jose and the South Bay. “Silicon Valley” is principally an economic/cultural regional name laid over Santa Clara Valley and adjacent areas, rather than some completely separate geological trench. This is where somebody says “the Valley” and might mean venture capital rather than Van Nuys.

Central Valley — the enormous one.

This is the roughly 400-mile agricultural trough through California's middle. Its northern third is the Sacramento Valley and its southern two-thirds the San Joaquin Valley; the southern San Joaquin portion includes the Tulare Basin.

And this creates California's finest linguistic trap:

“I grew up in the Valley.”

In Los Angeles: San Fernando Valley.

In Fresno: probably San Joaquin/Central Valley.

Around Sacramento: possibly Sacramento Valley.

In San Jose in a 1998 business meeting: potentially Silicon Valley.

In Napa: probably nobody says it because they are busy informing you which hillside their grapes experienced.

Coachella Valley = Palm Springs/Indio/La Quinta/Coachella. Desert resorts, golf, date palms, retirees, music festival. Not Valley Girls.

Napa Valley = wine. You knew this one.

Sonoma Valley = wine but wishes to discuss why it is not Napa.

Salinas Valley = agriculture and Steinbeck. Vast lettuce-related activity.

Owens Valley = spectacular Eastern Sierra trough and one of the foundational chapters in California's endless water wars.

Death Valley = the state finally named one honestly.

Imperial Valley = irrigated desert agriculture near the Mexican border. A physical demonstration that California looked at a desert below sea level and said, “We can grow lettuce there.”

Temecula Valley = Inland Empire wine country whose fundamental proposition is “What if Napa were next to a subdivision?”

San Jacinto Valley = Riverside County interior; Hemet/San Jacinto territory.

Mission Valley = San Diego. Much smaller, intensely urbanized, freeways/malls/hotels threaded along the San Diego River. Yet another legitimate valley that nobody means when they say The Valley.

So the cultural hierarchy is essentially:

San Fernando Valley → THE Valley → Valley Girls

Santa Clara Valley → Silicon Valley → Engineers

Central Valley → Sacramento + San Joaquin → agriculture

Napa/Sonoma → wine

Coachella → Palm Springs + festival

Death Valley → death

the remaining 280-odd → California continuing to name every elongated low spot between two hills.

(http://www.autoadmit.com/thread.php?thread_id=5905091&forum_id=2],#50143488)