Everybody asks about earthquakes, so let us cover this right away: In California the North American and The Pacific tectonic plates meet - at times in an unruly and shaking fashion. Somewhat simplified North America is trying to move north west and in this proces The Pacific Ridge, the mountain range that along the way forms the San Francisco peninsula, is pressed up. But the state is covered in bigger and smaller fault lines criss crossing all over.
The San Andreas Fault line was responsible for the two major San Francisco quakes in 1989 and 1906. But the fault line runs for more than 1.300 km all down the state to the heavily populated areas around Los Angeles. The 1994 Northridge quake near Los Angeles was attributed to another system of fault lines - San Andreas has been dormant in the southern part of the state for over 300 years. For the practical implications of earthquakes see Emergency Preparedness.
From west to east the topography of the state is The Pacific Ridge, then a very long range of valleys that contains San Francisco Bay and Sonoma Valley. Santa Clara Valley that forms the basin south of San Francisco Bay was formed when the mountain ranges rose around it, not through erosion. East of the Bay comes another ridge, then Central Valley, and finally Sierra Nevada, the mountain range that to the south ends north of Los Angeles and to the north stretches into Oregon. Sierra Nevada are real mountains 10,000 feet / over 3 km above sea level. Compared to the Sierras the other ridges are small hills even if they reach more the 4,000 ft at Mt. Hamilton east of San Jose. But they are still challenging to build roads across and consequently much of the Coastal Ridge is fairly untamed, covered in chaparral.
Sierra Nevada creates a natural barriere between the eco systems of California and those of the adjoining states. To keep out foreign insects and pathogens that could be detrimental to the Californian agricultural production, the authorities are very strictly stopping food from uncontrolled crossing the state line. In airports as well as at major roads into the state there are veterinary controls, and meat, dairy, fruit, and vegetables all have to pass by these watchdogs. Literally. You will very likely meet a nice well behaved dog at the airport - and it is not necessarily sniffing out narcotics but just as likely apples and bananas.
West of the Pacific Ridge it is often cloudy as the moist air is pushed up over the mountains. When most of the precipitation has been left on the western slopes the clouds roll over the peaks where higher temperatures normally disperse the clouds. The combination of cool nights and dry warm days gives the San Francisco Bay Area ideal living conditions. In Central Valley temperatures can regularly reach 40C in the summer as the land doesn't cool down as much during the night.
In the southern part of the state another wind pattern occurs regularly. When differences in pressure lead air from the central part of the state towards the ocean these winds become increasingly hot as they are forced through the narrow valleys. The Santa Ana winds are both hot and forceful and combined with the dry weather they can lead to self combustion of the vegetation and scary wild fires.
As a general rule there is no (or very little) rain from May through October. Rain comes is storm systems, rolling down from Alaska. So even during the rainy season there are rainy days intersected by clear days and sun. In the Sierras winter precipitation is usually in the form of snow - a good storm can leave 1-2 meters of snow and driving with chains is a frequent requirement under these conditions. The snow is not just for skiing - during the dry summer the runoff delivers most of the state's water supply by way of several huge reservoirs.
In the urban areas creeks and rivers are often lead through big concrete channels, many of which serve as very useful bike routes during the summer. If you only see them a that time of year (or in movies like Terminator II or Grease) it is difficult to imagine the amounts of water it takes to fill them up. But filled they get.