National Boards Dental Exam Part I - Tests you on the basic sciences you learned during the first two years of dental school such as Gross, Histo, Micro, Path, Biochem, Dental Anatomy, etc. Usually taken after 2nd year, but some schools take it earlier.
NBDE Part II - Tests you on dental topics covered since freshman year such as Operative, Fixed, Removable, Endo, Perio, etc. Usually taken during 4th year, but some schools take it earlier.
Both of these are just big giant multiple-multiple-choice exams that takes whole days to complete (1 day for the Part I, 2 days for Part II).
I wouldn't worry about it until a few months before the test.
HTH.
