Hello and thank you for writing.
This is the form email which I send to people who ask me to identify spiders based on text descriptions. I'm sure that you've done your best to describe your spider to me. Unfortunately there's just too damn many of them, and color, pattern, size, and degree of fear evoked just aren't as informative as we'd like them to be.
If you're curious about whether it's dangerous, there are only three spiders in the continental US which you need to worry about: the black widow, easily recognized by it's shiny black aspect and red/orange/yellow "hour glass" on its tummy; the brown recluse, of which there are no populations in California; and the hobo spider, also not known from CA.
For good information see
Internet searches will yield numerous other web sites of varying quality. Caveat emptor. THE WEB IS A FLAWED RESEARCH TOOL!!!
If you're really curious and need to know what kind of spider you have, I'll see what I can do if you bring it or send it to me at UC Berkeley:
201 Wellman Hall
Berkeley CA 94720
...although it may not be possible for me to identify immature spiders and I can't really describe mature ones to you in this medium. Specimens should be preserved in alcohol or alive, and MUST BE ENTIRE, NOT SQUASHED!!! Include the county, town, and date where & when the spider was found. Your spider will become the property of the Essig Museum of Entomology, otherwise I don't get paid.
Alternately a decent macro image might be useful, such as one taken with a coolpix 990, equvilant or superior camera. You'll need to tell me the type of web or microhabitat (under log, on blossoming wildflowers, etc).
If you want to learn more about spiders a good starting place is "Spiders and their Kin " by Herb & Lorna Levi. This is an excellent way to begin to learn how spiders differ from one another, but some of the taxonomy is out of date. If you want to learn to identify genera, you'll need a disecting microscope and either Kaston's "How To Know The Spiders" or Roth's "Spider Genera of North America." The latter is more complete but harder to use. Learning to make species determinations in all but a few cases will require the primary literature. Good luck!