http://collegesearch.collegeboard.com/apcreditpolicy/index.jsp
Per College Board's Program Guide:
Each AP Exam score is a weighted combination of the student’s score on the multiple-choice section and on the freeresponse section. The final score is reported on a 5-point scale. Although colleges and universities are responsible for setting their own credit and placement policies, AP scores signify how qualified students are to receive college credit or placement:
5 = extremely well qualified
4 = well qualified
3 = qualified
2 = possibly qualified
1 = no recommendation
Overview of AP Exam Scores
Advanced Placement Incentive Grant