"Rotations" generally refer to your 3rd and 4th year clerkships, in which you'll rotate through different dept's, i.e. fam med, int med, peds, surgery, ob/gyn, er, etc.
"Residency" is your post-graduate training, in which the first year is generally considered your "internship" year (some internships are separate from your residency - some are combined - this is a whole other thread, though)
"Fellowships" are usually post-residency training programs, normally 1-2 years, in which you undergo further subspecialty training, i.e cardiology, infectious disease, gastroenterology, etc. Of course, certain fellowships are only available to certain residencies, you obviously can't pursue a cardiothoracic surgery fellowship if your residency was in family practice.
(Sometimes, however, "fellowship" in used to describe additional trng w/in medical school, e.g. here at OUCOM we have primary care and OMM "fellowships" whereby you spend a 5th year as a student focusing on one of these areas)
I hope I didn't confuse you more.