The heart of a college application is the student’s academic record—the courses taken and the grades achieved in those courses. Selective colleges uniformly state that they are looking for students who show convincing evidence of being able to do well in a demanding academic program and that they place the greatest weight in admissions decisions on that record.
How Challenging Is Your Academic Program?
Many colleges provide students with guidelines about the kinds of preparation they expect successful applicants to have in high school—the number of years of English, mathematics, foreign language, and so forth. Williams College, for example, provides the following guidelines to prospective students:

Applicants should pursue the strongest program of study offered by their schools. Wherever possible, you should take honors or advanced level courses, especially in fields of great interest to you. A challenging and well-balanced program of study ideally should include: a full four-year sequence in English and mathematics; study of one foreign language for three or, preferably, four years; and three years of study each in the social sciences and laboratory sciences. These are not absolute requirements for admission; rather they are recommendations for developing a strong high school record.

Usually, these are minimum recommendations; additional years of a single foreign language or mathematics, for example, are viewed favorably. Colleges also expect students to take advantage of opportunities their school may offer to challenge themselves academically through honors or Advanced Placement (AP) courses, or by participating in the International Baccalaureate (IB) program. If opportunities for challenging classes are open to you, a selective college will expect you to have taken advantage of them.

AP courses are designed to allow students in high school to take individual college-level classes in subjects of their choosing. The IB is a rigorous two-year curriculum covering a range of subjects, also at the college level. Both are prized among college admissions officers at selective colleges, since they each culminate in rigorous subject matter tests scored by independent graders using calibrated standards. Strong performance in these courses (and particularly on the AP and IB tests themselves) indicates that a student can do college-level work. It has also become fairly common for students, particularly juniors and seniors, to take classes at a local community college or nearby four-year college in subjects where the students have exhausted their high school’s offerings. College admissions officers also favorably note these classes.

Students and parents sometimes ask if it is better to get an A in a regular course or a B in an AP course. My answer is that it is best to get an A in the AP course. - Comment by Harvard admissions officer at group information session, followed by nervous laughter from the audience
The bottom line is that a straight A record will not make up for a weak course load if your school offers the option of more advanced coursework. Good grades are important, but the rigor of your course load is even more important. Many high schools reward a student who has taken a challenging course with a weighted grade at the end of the semester. If an A in a regular course is worth four points, for example, an A in an AP class may be worth five points. Grade weighting sometimes extends to the pluses and minuses that a student may receive as well, so that an A+ in an AP course might be assigned a total of 5.25 points, while an A– in such a course might be worth 4.75 points. The weighting of honors, IB, and AP classes can result in some astronomical grade point averages (GPAs) for students who take heavy loads of such courses and do exceptionally well in them.

If your school offers AP classes, you look bad not taking them. You shouldn’t take so many that you get bad grades, but you need to challenge yourself. - College freshman reflecting on the high school experience
It is sometimes disheartening for students to learn that some colleges recompute each applicant’s GPA in unweighted form, including only the years (usually tenth, eleventh, and twelfth) and classes (usually academic “solids” such as English, foreign language, math, science, and social studies) they wish to consider. Admissions officers do this to have a common standard for discussion. But despite any recalculation, due consideration will be given to the nature of the courses you are taking, within the context of the opportunities you have had. A high GPA in a weak curriculum will definitely be less well received by a selective college than a somewhat more mixed record in a challenging one. In addition, two students may have similar GPAs in comparable courses, although one has shown improvement over time while the other has grades that have been declining. The evaluation of these two students’ academic records will be very different; improvement over time will be viewed much more favorably by admissions staff. This is another example of how the evaluation process considers an applicant’s record in context.