There’s a very good explanation for what the function of the CDER (Center for drug evaluation and research) at the FDA’s own FAQ page found here:

http://www.fda.gov/AboutFDA/CentersOffices/OfficeofMedicalProductsandTobacco/CDER/FAQsaboutCDER/#1

Wikipedia also has a good definition:

CDER reviews New Drug Applications to ensure that the drugs are safe and effective. Its primary objective is to ensure that all prescription and over-the-counter (OTC) medications are safe and effective when used as directed.