Infractions are the least serious type of crime, like, moving violations, non-moving violations, and other petty offenses.
Misdemeanors are offenses above infractions in terms of seriousness. In US federal law, they are ranked in classes: Class A, B, and C misdemeanor. Generally, such offenses are penalized by jail time.
A felony offense is the most serious type of crime. State and federal law call for prison punishment for felonies, but varies in length from state to state, etc.
Felony offenses are also grouped in classes: A, B, C, D, & E with A being the most severe and could be punished by death penalty (or whatever is the most serious punishment.