It's different for different specialties.
There are 3-4 different varieties. Some specialties (Anes, Rads, PM&R - I think, etc) don't start until your 2nd year after graduation (post-grad year 2 or "PGY-2"). At the same time you apply to these programs (4th year of med school) you are applying to programs for your intern year. These can take 1 of three forms:
Medicine Prelim: You are an intern in an internal medicine residency program. Your schedule is similar to the "categorical" (read- matched for 3 years) interns (MICU, floors, clinic). After your 1 year you leave to go on to your regular specialty. Popular with Rads, Anesth, etc.
Surg Prelim: similar to the medicine setup but you are doing what the surg folks do (round on floors, do clinic, cover trauma, occasionally see the OR). Often necessary for Neurosurg, Uro, ENT, etc.
Transitional Year: The cush intern year if you do it at the right place. This is much more of the traditional "rotating internship" where you do peds, OB, medicine, surg, etc with usually quite a bit of elective time. Because hours and schedules tend to be a bit better than either of the above these tend to be more competitive.
Now, if your program does not start in the PGY-2 year (and examples of ones that don't would be Peds, most EM, Med, FP, Gen Surg, OB) your intern year is basically just rotating through all the services of that specialty. So obviously the difficulty of it is based on the a) specialty, and b) hospital you are at. For example, surg may be nasty at one hospital and mostly bearable at another. Or an EM internship may involve Neurosurg ICU, Trauma ICU, PICU, MICU at one place but only one unit month at another. So it really depends on the residency.
Since you posted in FP I'll let one of them answer, but overall in my experience FP internships tend to be like the transitional year - you do a bit of everything.
C