In California, legal separation differs from divorce in that it does not change one's marital status and does not allow one to remarry or start a new domestic partnership. However, legal separation often becomes a foundation for a later filing of divorce.
Many suppose that legal separations are temporary while divorce is permanent, but that is not really the distinction. It is entirely possible for the status of being legally separated to continue for years or even a lifetime. Typically, however, legal separation is pursued when one is not sure if final termination of the marriage is the course he/she wishes to take.
Legal separations involve negotiations over property ownership, responsibility for debts and bills, child custody, visitation, and many of the same things divorce negotiations involve. They also frequently entail a petition for a restraining or protective order. Finally, note that another reason for getting a legal separation (at least initially) is that it will take effect immediately - there is no 6 month waiting period like most divorces require to be finalized.