Ask someone from California where Southern California ends and Northern California begins, and you are likely to get a clear answer. But ask an entire group of Californians the same question, and you'll likely witness a lively discussion.
The internet's answers vary from 'Anything other than the southernmost 10 counties are NorCal' to 'If you see Birkenstocks you're in NorCal, if you see flip-flops you're in SoCal.' It seems that various groups and organizations divide the region into North/South at different points and for different purposes.
Some of the disagreement is simply due to California's large size. Many think of the Central Valley as being a separate region from the northernmost parts of the state. In fact, residents of the extreme north have even attempted to form a new state. Still others have suggested California should be split into three separate states due to geographical and cultural differences. With all these points of view, we are still left with no clear definition of 'Northern California.' But, because we like simplicity, we are choosing to follow the majority and define the region as the northernmost counties of the state, from Sacramento to the border with Oregon.