The differences come in sentencing. Different crimes fit in different "brackets," and those brackets determine the range of sentencing. First degree murder would be Class A felony, while Manslaughter or Second Degree Murder, rape, and other violent crimes would likely be Class B felony. It scales down similarly from there.
The same goes for misdemeanors, although they are usually just A & B classes. A would potentially encompass a year in jail + a $1000 or so fine, while B would be limited to 6 months in jail and $500 fine. These are just examples, and may vary from place to place.