Both of the above replies are inadequate in describing the differences between grad, undergrad, and post-bacc as defined by AAMC. I would refrain from using course numbers as they are not standardized among the thousands upon thousands of schools in the US. What Instatewaiter described in terms of lower level classes would be graduate school (200-level), law school (300-level), and medical school (400-level) courses respectively at UC Davis. Clearly not the same.
Anyway, this is what these classes mean:
1) Undergraduate: For the most part, these are classes that are normally taken for a bachelors degree (e.g., undergrad degree).
2) Graduate: For the most part, these classes are normally taken by graduate students (e.g., masters and PhD students). HOWEVER, professional school courses also fall into this category. This includes med school courses.
Therefore if you were PhD student in Chemistry, and you take Physical Organic Chemistry--that is a grad level course. If you take Clinical Pharmacology through a med school, it will also be considered as graduate level. It is true that you do this after receiving your undergrad degree, but microgin's definition can also be confused with post-bacc.
3) Post-bacc: Although the OP did not include this, i believe it would be good to talk about this. Post-bacc, as defined by AAMC, are any UNDERGRADUATE level classes taken after receiving your bachelors degree. This includes if you do a 2nd, 3rd...etc bachelors degree. This also includes taking undergrad level classes while doing a masters or PhD.
In the grand scheme of things, when you apply to med school, these are seperated out into undergrad GPA, overall undergrad GPA (undergrad GPA + post-bacc GPA), and graduate GPA. So thats why its important to know what the differences are.
Therefore, when they say "graduate credit" (like in the program in the link), then that means you are taking graduate level classes as defined by #2, and thus you will be adding to only your graduate GPA. If it is aimed at boosting your undergrad GPA, then they will have classes that give you "undergraduate credit" which would include post-bacc. Hope that clears things up. Good luck!
