A 90 is a A- and is one percentage point above a B+. 90 is the lowest percent grade in the "A" letter range.
Aside from 90 out of 100, there are numerous ways students can end up with 90% grades. Here are scales for common number of test questions that can lead to a 90 grade.
If you're consistently getting grades around 90% on your tests and assignments, you're on a great path towards getting into really good colleges. Check out the complete discussion on 3.7 GPA to compare your grades, or jump right to the list of colleges you can start thinking about.
Reference table of the percentage grade scale with letter grade and unweighted GPA point equivalents. This grade scale is used my most U.S. middle schools, high schools and colleges.