Are non-compete agreements enforced in the state of California?

Most states enforce noncompetes, and when they do, they are enforceable as per their plain terms, absent "overreaching." By that, I mean that since the purpose of a noncompete is to prevent an ex-employer from being harmed by competition from its ex-employee, a noncompete can generally restrict you from working for an actual competitor (so someone selling the same goods or services to the same customer base) for a reasonable period of time (generally between 6 months and one year). If it is broader, longer, etc. than that and unfairly restricts your ability to work at all, a court will generally--if the ex-employer sues you to enforce the agreement--cut it back (often called "blue penciling" it for some reason) to more reasonable levels. To the extent a noncompete is valid and enforceable, if you violate it, you can be sued for monetary compensation and/or a court order barring you from working for the competitor or competing in your name/with your own business.
CA will not generally enforce noncompetes, so IF you try to compete in CA (work for a CA competitor), the odds are good the court will not enforce. That will not help you if you compete in another state, however. And there are certain "non-compete-like" agreements that CA will enforce, like those barring you from performing work for the ex-employer's customers or clients.