Fellowships are for extended specialty training. They are not required to practice medicine per se, but if you want to be a cardiologist, oncologist, pediatric endocrinologist, pediatric surgeon, vascular surgeon, gynecologic oncologist, interventional radiologist, dermatopathologist, etc. (the list goes on essentially forever but basically it includes anything but an internist, pediatrician, general surgeon, OB, Family doc and few other things) then you'll need to do a fellowship. They range from 1-4 years in length, some portion of which will be spent doing research in most academic programs. This will make your total training anywhere from 4-10 years depending on how you decide to roll.