Quick Answer
Cumulative GPA is your average across every semester and course you've completed, as opposed to a single semester's GPA.
Cumulative vs Semester GPA
Your semester GPA reflects only the classes from one term. Your cumulative GPA averages every grade you've earned across your entire academic career at that school - it's what colleges and scholarship committees typically request.
To calculate it, add up the grade points from every semester, then divide by your total credits across all terms - not by the number of semesters. Use the cumulative GPA calculator to do this automatically across multiple semesters.