In an effort to acknowledge the effort that goes into challenging courses, many high schools weight the grades for AP, honors and accelerated courses. The weighting isn't always the same from school to school, but a typical model on a 4-point grade scale might look like this:
AP, Honors, Advanced Courses: 'A' (5 points); 'B' (4 points); 'C' (3 points); 'D' (1 point); 'F' (0 points)
Regular Courses: 'A' (4 points); 'B' (3 points); 'C' (2 points); 'D'(1 point); 'F' (0 points)
Thus, a student who got straight 'A's and took nothing but AP classes could have a 5.0 GPA on a 4-point scale. High schools will often use these weighted GPAs for determining class rank -- they don't want students to rank high just because they took easy classes.
Colleges, however, usually aren't going to use these artificially inflated grades. Yes, they want to see that a student has taken challenging courses, but they need to compare all applicants using the same 4-point grade scale. Most high schools that use weighted GPAs will also include unweighted grades on a student's transcript, and colleges will usually use the unweighted number.
The GPA confusion doesn't stop here. Colleges also want to make sure that a student's GPA reflects grades in core academic courses, not a bunch of padding. Thus, a lot of colleges will calculate a GPA that is different from a student's weighted or unweighted GPA. Many colleges will look just at English, Math, Social Studies, Foreign Language and Science grades. Grades in gym, wood working, cooking, music, health, theatre and other areas will not be given nearly as much consideration in the admissions process.