It really depends on school/country. For example, in most Canadian schools:
0-49 F (Fail)
In graduate programs, the minimum grade for a MSc course is usually a C (the split above is still the same, except the grades C- and D are replaced with "F". For a PhD program, the minimum grade is a B- (i.e. anything below 67% is a Fail).
Canadian schools that decide to also use a GPA system generally adopt a 4.3 scale, where A+ is 4.3, A is 4.0, A- is 3.7 etc.
Just to give you a sense of what is considered a "good" grade in Canada (since it seems like our scales might be different):
- Letter grades in Canada are not usually assigned on a curve. That is, an "A+" doesn't mean "top 5%" or whatever. Instead, as you can see from the above scale, letter grades are assigned based on meeting some minimum criteria. So, it's possible (but very rare) for an entire class to get A+ grades. Generally, in the upper level Physics courses (which only serious Physics students take), the course averages are usually in the 75% to 85% range.
- The top Canadian graduate schools have grade cutoffs somewhere around an A- (80% minimum) average to be admitted, where the average is computed only on your 3rd and 4th year courses in your field (generally Physics, Math, and maybe Astronomy courses). The real competitive GPA is probably closer to 85%.
- To win graduate level fellowships, competitive applicants have GPAs in the high 80s although many top students will have GPAs above 90%.