GPA, or Grade Point Average, is a method of measuring the success of a student based on his or her grades. There are a few different ways to calculate GPA, all of which are outlined here.
Most schools use a four-point system to calculate GPA. Usually under these, an A is worth four points, a B is worth three points, a C is worth two, a D is worth one, and and F is worth zero. The scores are then typically tallied up and divided by the number of classes that the student is taking to get the average (this can be altered slightly if a student is taking classes worth different numbers of credits). The result is then the "grade point average" out of four.
Some schools take this a step further and have different values for A, A-, B+, B, and so on. A- and B+ are typically somewhere between three and four points. This tends to be around 3.3 for a B+ and a 3.7 for an A-.
Some schools use a 100 point GPA system rather than a four-point system. In these, anything above a 90 is A-level, 80 or above is B-level, 70 and above is C-level, and 65 or above is D-level. Usually when reporting your GPA, you would convert it to a four-point scale, as that is much more common and usually the preferred method of calculation.
GPA is a very common way for colleges, graduate schools and even employers to determine your ability and intelligence as a student. In fact, it is very important in determining college admissions for high school graduates. The top colleges and universities in the US tend not to accept students with a GPA below 3.5 (which is an A/B average). Anything above a 3.0, however, us usually considered successful.