Rotations, aka clerkships, are done during the clinical year (at most med schools during the 3rd year), as well as the 4th year...when students talk about being on rotations, this implies the 3rd year since 4th year is usually made up of sub-Is (where you act as an intern), away rotations, and electives. The required 3rd year rotations are surgery, internal medicine, ob/gyn, pediatrics, psychiatry, and family medicine (though I think I've heard of a few schools who have outpatient IM instead of FM). Neurology is also a required rotation at many schools. These clerkships last anywhere between 3-8 weeks long, and at the end of each rotation there is a test, which can either be a departmental exam or a nationally standardized test called the NBME shelf exam. Your grades are comprised of evaluations from residents and attendings and the exam at the end (which usually makes up 10-50% of your grade). Some rotations will also require things like formal presentations to be included in your grade. These rotation grades are widely considered one of the more important components of your application.
As far as electives go, this varies by school. Some will have all their elective months in the 4th year while others will have some elective time during 3rd year. My school, for example, has two 2-week blocks and one 4-week elective block during the clinical year. That's about all I can think of at the moment. Did I answer your question?