still not following
college credit: college and university are used interchangeably between most people in the United States. Taking certain classes in high school, college for many Euro folks, gives you credit at many universities as if you had taken the course there.
AP: Advanced placement courses. These are the high school classes that most universities will accept as credit for their own courses if you pass with a certain mark. The idea being you don't have to take introductory courses again in university. AP courses are graded on a scale of 1-5 and universities require a score of 3 or 4 typically to count it as credit. AP courses require much more effort and work to complete than the regular classes in high school, and often are more intensive than the university equivalents as well.
GPA: Grade point average. Basically your grade from school converted to a scale of 0-4.0 at most schools. Grading policies can vary from school to school so this gives a consistent scoring system that is used, especially by universities.