An infraction is a crime, albeit a low-level one. It is less serious than a misdemeanor, which is in turn less serious than a felony. (Note that some states consider certain kinds of infractions like traffic tickets to be civil, rather than criminal, offenses.)
Infractions typically don’t carry any jail time, but they often involve fines. The quintessential infraction is a traffic ticket, like one for speeding. (Some traffic offenses can be misdemeanors, and even felonies depending on the circumstances—DUI and hit-and-run are examples.)
For more on crime classification, including infractions, see Felonies, Misdemeanors, and Infractions: Classifying Crimes.