The MBA is a graduate degree, meaning that a bachelor's degree (not necessarily a BS) is required to enter an MBA program. The BS is an undergraduate, entry-level degree program for those with no previous university experience.
The MBA focuses narrowly on subjects relevant to running a business, such as accounting, leadership, project planning and advertising. The BS focuses on a particular science (such as chemistry or physics) but also includes a general education component with classes such as English composition and a foreign language.
Most BS programs require four years of full-time study to complete. MBAs are typically designed to be finished in two years of full-time study.
The BS is an entry level degree program, meaning that students do not need any previous college-level study to apply. Most MBA programs require a bachelor's degree as well as significant real-world work experience of their applicants.
BS holders can work in many industries directly, continue for a master's or doctorate degree in their field, or even apply for an MBA program. MBA degrees are typically pursued by those with a strong interest in working in business or finance.