Wow, your system seems so complicated and involved, but I guess that cause I haven't been through it. Anyways, probably the biggest difference in the US education system is that there are no national exams required to graduate from high school. The carriculum for a high school degree is determined by the individual states, some which will require you to pass standardized exams administered by the state's education board. What courses you have to pass is also determined by individual states. To get into an undergraduate college, you need to have a high school diploma or equivalent, and in most cases you need to have taken the SAT. The SAT is a standarized exam offered by an indipendent company which tests english and math comprehension. (There is also another test called the ACT which is equivalent to the SAT. Colleges typically don't have a preference on which one you take. Students in different regions typically take one or the other, the SAT's are prevalent on the east and west coast, while the ACT is prevalent in the central regions, though you can take either test anywhere in the US). College admissions is based off your performance in the standardized test, high school GPA, extra carricululars, and stuff like that. The choice of major is up to the student, but admission into colleges is typically based on the student demonstrating some interest or strength in that major. Once in college, you begin preparing for med schools. Almost all US med schools have the same pre-requisites, which is a year of physics, biology, inorganic chemistry, organic chemistry, and english, though there can be a little bit of variation on that. Additionally, you are required to have taken the MCAT, a standardized exam administered by the association of american medical colleges (AAMC) which tests knowledge in biology, chemistry, physics, organic chemistry, reading comprehension, and writing (there's an essay section). Getting into med school is primarily based on your GPA and your MCAT scores, as well as other items such as medical experience, extra carriculars, letters of recommendation, and so fourth; in a lot of ways, it's similar to applying to the undergraduate colleges. The highest GPA one can attain is %100, though it is commonly refered to in terms of a relative scale where 4.0 is the highest. Letter grades are assigned to courses taken in college which correspond to a numerical grade which averages into the GPA. (A = 4.0, B= 3.0, C=2.0, and so on. Thus, if you took four classes each of equal credit hours and got two A's, a B and a C, you would have a 3.25 GPA for the semester). Some colleges have slightly different GPA systems (MIT works off of a 5.0 system, the highest GPA you can get at Cornell is 4.3/4.0 if you get all A+ in every class). To apply to medical school, you use a service offered by the AAMC called the AMCAS application. It's a common application where you fill out all your information and designate which schools you want to have it sent to. The only limit on schools you can apply to is how much money and time you're willing to put into the process, as it costs $150 for the first school, and then $30 for every subsequent school. Not ever school uses AMCAS; for example, the Texas schools have their own small version of AMCAS, and Columbia uses its own application process. After schools receive the AMCAS, they have the option of sending you a secondary application in which you fill out some more information, write a few more essays, and send even more money (anywhere in the range of $50 to $100 per secondary). Then after you return the secondaries and have your letters of recommendation sent, schools may invite you for an interview or decide to reject you. Interviews are typically held at the school, though some will offer regional interviews in your area with a school alumni if you request it or have good reasons why you can't go to the school. After the interview, the school can choose to reject you, waitlist you, or accept you. If you're waitlisted, it means that you're on a list of potential candidates they'll offer acceptances to if sufficient people who were offered first round acceptances turn them down. You can potentially be called of a waitlist right up until the morning classes start.
Whooo, that's a lot of typing, I wish the words would flow that easily on my med school essays.
Hope I answered your question.