Germany uses the basic grading system of 1-6 (best to worst). Getting a 1 is amazing and 5 and 6 are usually considered unsatisfactory and do not meet the requirements for passing. Students can get stages between these grades, listed as + or -, or as a decimal (e.g., 1.3).
In America, basic grading is a little more complicated. A-F (without an E). A is amazing and F is a complete fail. In many cases, D is also considered failing. To calculate the Grade Point Average (GPA) each letter grade must be changed into a number (A=4, B=3, C=2. D=1. F=0). Some schools have different scores for plus and minus, others say that it does not matter if someone has a B- or a B+, it is all worth 3 points.
When grades are transferred from one system to the other, it must be decided what is considered "equal." Typically, grading rules say that an average grade in Germany is harder to get than an average grade in America. For this reason a German 2 is usually transferred into a A-/B+, a 3 is a B/B-, and a 4 is an American C.
When she studied in Germany she found it hard to get excited about "good grades." A 1 or 2 just did not sound very exciting when she grew up striving to get As. It was also a little strange to have to explain to people that a 3.66 GPA meant that you did very well, not that you were close to failing!
Funny how you can get such an emotional attachment to a few words and their meanings :)