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. If you get into a fistfight you could be charged with misdemeanor battery. If you really hurt someone badly in a fistfight, you could be charged with felony battery.