Deferring admission depends on what medical school you will be admitted to. Some do not permit derferral at all, some have a limited number of deferrals and only for what the medical school considers acceptable reasons, others are fairly generous with deferrals. Most will hold your position for a limited time within which you must tell them you are coming. Some have no problem if you want to reapply elsewhere, others have a strict policy of notifying AMCAS if you break the deal and you may be barred from reapplying anywhere the following year.
If you have a premedical advisor, she/he may have a manual (available exclusively to advisor members of the advisor professional association) which has information on policies for each school. Otherwise, you may have to call Admissions at any schools you are considering application to.
Nothing is simple.