The United States uses an amazing number of different systems. There is nothing standardized.
It is common to have "letter grades" in which A, B, C, and D represent a quality scale, and other letters stand for words and aren't part of the scale. F means "failing." There isn't any "E" because F isn't part of the quality scale. Another common letter grade is "I" for incomplete. It means what it says, and typically the student has the opportunity of finishing the work, in which case the "I" in the record is replaced by a letter grade.
In my life, I've been graded in all the following ways:
1) a numeric scale of 0 to 100 (with 60 considered "passing;")
2) word grades of excellent, very good, good, fair, poor;
3) "pass/fail," which means what it says; you either pass or fail, and no distinction is made between degrees of performance if you pass;
4) letter grades with plus signs, i.e A+, A, B+, B, C+, C, D+, D... and F. (Again, since F is not really part of the scale, there isn't any F+."
5) letter grades with both plus and minus signs: A+, A, A-, B+, B, B-, C+, C, C-, D+, D, D-, F.
At universities, however, it is customary to calculate a cumulative grade-point-average or GPA, often on a scale of 0.0 to 4.0, with 4.0 ("four-oh") being the highest possible. This often means that the professors file their results as letter grades, which are then converted to numbers (A = 4, B = 3, etc.). Cumulative means that the number on your transcripts at the end of your third year includes grades for all three years.
Many professors grade on an absolute standard--a certain level of performance is an A, and if everyone in the class is very good, everyone in the class gets an A. Occasionally, a professor will "curve the grades." This means that grades are assigned according to their weights in a statistical "ball curve." That means regardless of scores, the top 20% of the class will get A's, the next 30% B's, or some such system.