Peninsula
Lying immediately south of San Francisco, the Peninsula continues southward through San Mateo County and into the northern part of Santa Clara County. It has two coasts: one on the Pacific and the other on the San Francisco Bay. Although there are few major cities here, it is a beautiful area. The Santa Cruz mountains split the region in half from north to south, and there is a large swath of wilderness that makes for prime hiking and biking.
South Bay
Situated south of The Peninsula and East Bay, this region encompasses most of Santa Clara County and is home of the most populous city in the Bay Area, San Jose. It is commonly referred to as Silicon Valley due to the large number of high-tech companies headquartered here.
East Bay
Across the Bay Bridge and east of San Francisco you'll find East Bay. Comprised mostly of Contra Costa County and Alameda County, it is home to the city of Oakland, its largest city, as well as Berkeley.
North Bay
This area encompasses Marin, Solano, and Sonoma Counties, as well as the Napa Valley, which is world renowned for its wine production. Located at the northernmost part of the Bay Area, it lies to the north of the iconic Golden Gate Bridge.