NorCal is the current slang (At least in California colleges) for Northern California.
The idea of NorCal and SoCal is less of a physical separation rather than a culture separation. Most particularly, a lot of NorCal people (Or maybe just teens?) say, "hella" (i.e. That's a hella cool!).
It could be argued that NorCal culture tends to be perceived to be more cultured than SoCal which is defined by Los Angeles and the mess that is the freeway system, the movie industry, and various other stereotypical traits that tend to make the culture of SoCal be perceived as vain and facetious. NorCal, is, on the other hand, defined by San Francisco. It has the Golden Gate Park with its myriad museums and splendor, a blossoming artistic community (Not to say that Los Angeles doesn't have one, it's just not as propagated), and some interesting quirks.
If I were asked to define a physical boundary of these 2 ideologies, I would have to say the end of SoCal is Santa Barbara, and NorCal starts around Santa Cruz or the Monterey Bay area in general (I think Carmel-by-the-Bay is a bit of a push...after all, Clint Eastwood was its mayor once, and yes, I know he was born in San Francisco). Central California is too sparsely populated to have a real definition...unless you just want to call it farmland...(I'm kidding!)
* This is speaking from someone who's lived in Los Angeles their whole life...