Postbacc-- usually for those with an average GPA/a 3.3-3.7, to help bring your GPA up. Any units taken will be added toward your undergraduate GPA.
Masters -- usually for those with a low GPA/3.2 & below, to help show dental schools you can handle a rigorous science courseload. This results in a grad GPA separate from your undergrad GPA, and you get a masters degree.