Infractions, sometimes known as regulatory offenses, are the least serious class of crime in the criminal law system of the United States. Many infractions are not even serious enough to be considered crime, and are instead often treated as civil cases and handed over to civil courts. Infractions are, in general, similar to civil cases in that the prosecution need only demonstrate "a preponderance of evidence" to prove the defendant's guilt, rather than beyond a reasonable doubt, as is required by criminal trials for misdemeanor and felony charges. Nevertheless, infractions are technically crimes, inasmuch as they are violations of statutory law that affect society as a whole. (You're hurting society as a whole when you jaywalk!)
Infractions are never the cause of any formal social stigma, and are punishible by fine only, never incarceration. It must be noted, however, that in many jurisdictions a series of unaddressed infractions may lead to a misdemeanor charge and accompanying incarceration. For example, parking violations are punishable by fines only, but multiple unpaid parking tickets may result in a misdemeanor charge and subsequent incarceration until the tickets are paid.
Jaywalking, littering, building code violations and disturbing the peace are a few examples of common infractions. Some states have even passed drug laws that make variying degrees of posession & use an infraction. For example, possession of less than an ounce of marijuana in Oregon is considered an infraction rather than a crime, and carries no serious consequences.