There are different kinds of crimes. Some are much more serious than others are.
Felony crimes are more serious than misdemeanor crimes.
If you are guilty of a felony, you can be locked up for a much longer time. The maximum time you could be locked up for a misdemeanor crime is 1 year. The maximum time you could be locked up for a felony crime is many years, possibly even until you are an adult. Some crimes can be either a misdemeanor or felony, depending on how bad the behavior was. A fistfight is one example.