Its a bit more complicated than that, as graduate programs have pre-reqs and entrance exams that need to be taken similar to your SATs for undergrad. Also, some graduate programs like an MBA require/prefer work experience. Some grad program's are shorter as well, and may give preferential admission to currently enrolled students (which appears to be a seamless continuation of your undergrad curriculum).
The general progression is BS (undergrad, ~120 credits), Master's (graduate, 30-60 credits or 1 to 2 years), PhD (doctorate, varies greatly but requires lots of research in place of actual classes).
Chiz