Every region has its lingo, and the Bay Area is no different. People routinely refer to East Bay, the Peninsula, South Bay...but what exactly do they mean? Here's a quick overview to help you sound like a local.
Peninsula - Area code 650. Includes anything south of San Francisco down to Mountain View. This area is east of highway 280, on the west side of San Francisco Bay. San Mateo and Santa Clara counties.
North Bay - If you cross the Golden Gate Bridge, you are in North Bay. Mostly Marin County; shares the 415 area code with San Francisco. Farther north is Sonoma County.
East Bay - Once you head east over one of the three bridges (Dumbarton, San Mateo or Oakland/Bay Bridge) that cross San Francisco Bay, you are in the East Bay. Oakland, Fremont and Berkeley are all East Bay. Mostly area code 510. As you move further inland from the Bay, south and east of Oakland, there is an area sometimes referred to as the Tri-Valley region. It includes the cities of Pleasanton, Dublin, Livermore and San Ramon.
South Bay - Anything south of Mountain View, up to and including San Jose and Milpitas. Generally area code 408. Mostly Santa Clara County.