Med schools in the U.S?Tru Calling.?

What are Mcats? do those who get into med school take them just after they finish high school or after they finish premed school? what started the whole thing was that i was watching Tru Calling the tv show and Tru is a college graduate who wants to take Mcats to get into med school.She already graduated from college y does she want to get into one again?my question is if some1 wants to be adoctor,what do they do in the U.S? what exams do they take?What's premed school?how long to u stay in there?what happens after it's over?how long do they stay in med school?