Report cards show how well you did during a grading period, which can comprise a quarter, trimester, or semester--each of which constitutes several months. One quarter or trimester typically is comprised of 1.5-2 months accordingly, whereas a semester is comprised of one whole half of a school year. There are two semesters (Fall Semester, Spring Semester), four quarters (two in the Fall Semester, two in the Spring Semester), or six trimesters (three in the Fall Semester, three in the Spring Semester). Your quarter or trimester grades are determined based on how well you did in each of your classes during those select 1.5-2 months of learning, while your semester grades are determined based on your overall grades for each of your classes throughout the past three trimesters or two quarters therein, respectively. Your overall grades for the entire school year are averaged out based on how well you did at the end of each of the two semesters. If you did better during one semester over the other (either one), your final grades for the academic calendar year will be averaged out and ultimately determined on your report card based on the higher sets of your separate semester grades.
I hope this answers your question. Good luck with the homestretch of the school year, Kenn!