In High Schools letter grades are equivalents of percentages, after that College and Universities you need credits to pass a subject.
A 93 - 100% Excellent
B 85 - 92% Good
C 77 - 84% Average
D 70-76% Passing
E 55-65% Failing (but eligible for summer school)
F 0-54% Failed not eligible for summer school
There is a lot of information out on the web on credit systems in colleges and Universities (North American system, not sure about Europe and if they have the same thing).