Mr. Tappers seems to be doing a great job in here, but I thought I'd butt in for a second.
The standard "traditional" curriculum for the 4 years of medical school is 2 years of pre-clinical sciences (think 2 more years of science class, like college again) followed by 2 years on the wards doing your rotations (working in hospitals as part of the medical care team). Of the two clinical years, the first (year 3) is often "core" rotations (peds, IM, family med, gen surg, psych, obgyn) and the second year (year 4) has more elective time. Of the pre-clinical years, the traditional format was anatomy and physgiology in the first year (year 1), and then second year (year 2) delves into pathophysiology, which is the study of physiology in a diseased state.
The idea was you learn the human body in year 1, then learn about sickness and disease in year 2, then put that to work in the hospital year 3, and then focus on what you really want to do in year 4.
Some problems with this system was that it is really hard to all of a sudden transition from book learning to working with real people in a hospital. Therefore, basically every US medical school is now smushing and mixing all the years together, so that the disticntion between "pre-clinical" and "clinical" years are now very blurred. Some schools have even gone so far as to do a 1.5 year "condensed" pre-clinical years system (Baylor, Vanderbilt, UVA, more as well) and some even have a 1 year (!!!) pre-clinical curriculum (Duke, Vanderbilt for the class of 2017 and later, maybe more).
However, basically every school will have you working with either standardized patients or maybe even real patients your first year, and many schools now have student-run clinics where students can get great hands-on experience starting the first few weeks even.
In response to your second question, I would like to leave you with two thoughts. The first thought is that admissions commitees really like to see dedicated involvement to activities, and not jumping around all the time. That would mean that 3+ years of an involved activity is most likely seen as more valuable than 5 different activities each only a semester long.
The second thought, however, is that in the context of your hospital volunteering, unless you are really establishing some relationships that you could see developing your volunteer involvement to a greater level (ie they give you more responsibilities, let you do cooler, more interesting stuff since they trust you more), seeing more of the hospital might give you more stuff to explore and talk about in your application. You might find something you're really interested in as well.
P.S. If you're interested in my experiences with the application cycle, you can see a quick preview through my MDapps which is linked under my name.
Good luck to you all and I'd be glad to answer any more questions you might have, although Mr. Tap seems to be doing a mighty fine job with it already.
