Bike routes in California (and in most parts of the U.S., I believe) are pretty much the same thing that DataJunkie and wahoonc described in their states: they're the city or county's suggestions for good roads to ride on. Sometimes they're excellent suggestions, other times they're flat-out lousy...and no two riders will always agree on which ones are which. In any case, you're certainly never limited to riding on official "bike routes"; take them as a suggestion, but choose the roads that get you where you're going the most smoothly, safely, and enjoyably.
To address your question about riding on sidewalks -- that's almost never a good idea. Sidewalk riding may or may not be legal where you live, but it's almost always inefficient and unsafe (not just because you may be endangering pedestrians, but because cars which turn across your path at driveways and intersections won't be expecting to encounter a fast-moving cyclist on the sidewalk, and you and the driver often will lack a good line of sight to one another.)
By the way, the Advocacy and Safety subforum has a long thread on the question of sidewalk riding; you can find zillions of opinions there, links to relevant studies, and advice on those (rare) times when it really IS better to take the sidewalk.
Good luck, and enjoy your ride!