Crimes generally fall into one of three categories: felonies, misdemeanors, or infractions. Out of these, infractions are the least serious and punishable by fines only (no jail time). Probably the most well-known infractions are traffic offenses, but infractions can also be things like loitering, jaywalking, public intoxication, or littering. States may also call infractions by different names, such as violations, petty offenses, or petty misdemeanors.
State law defines offenses and their penalties. Infractions are fine-only offenses, whereas misdemeanors (the next step up in severity) can be punished by fines and jail time (usually up to one year's incarceration).
States draw the lines differently between infractions and misdemeanors. For instance, possession of marijuana can be an infraction in some states but a misdemeanor in others. Most states classify traffic offenses, like speeding or running a stop sign, as infractions but more serious traffic violations, like driving under the influence or reckless driving, as misdemeanors.
Often with an infraction, you can pay the fine and be done with it. If you want to fight an infraction ticket or citation, however, you will have to go to court. Many states have traffic courts that only handle minor traffic citations, or you might need to appear in criminal or civil court.
Defendants facing infraction charges generally receive less procedural protections than defendants facing charges for more serious offenses. Because infractions don't carry the threat of jail, a defendant doesn't have a right to a jury trial or a government-paid lawyer. You may hire a lawyer to help you fight an infraction or traffic ticket, but the government will not appoint one. A judge will hear the case and determine guilt. Depending on the jurisdiction, the government might need to prove the offense only by a preponderance of the evidence—a lower burden of proof than in criminal cases, which require proof beyond a reasonable doubt.
Some states handle infractions in civil, not criminal, court. The advantage of a civil infraction is that there won't be a criminal record for the violation. However, civil court provides less procedural protections than criminal court, and the government has a lower burden of proof in a civil case.
While most infractions are minor violations, they can escalate into more serious situations. A routine traffic stop for something small, like failure to signal, can lead to a search of the vehicle if the officer has reasonable suspicion of criminal activity. And that search could lead to arrest and potentially more serious charges.
If a driver racks up a series of infractions within a certain time frame, the violations can become misdemeanors instead of infractions, and at that point, jail becomes a possibility. In some states, traffic violations add points to your driver's license record, which can result in license suspension or an insurance rate increase.
Ignoring court orders and fines can lead to more serious charges as well. If you cannot afford a fine, don't ignore it. Talk to the court and tell them your situation; many courts will institute a payment plan or work with you. Otherwise, a judge can issue a bench warrant directing you to appear before the court and explain why no payment has been made.
Infractions can often be handled without a lawyer. But, if you think a police stop was improper or the infraction becomes a more serious charge, talking with a lawyer could help resolve the situation quickly and with the least impact to your criminal or driving record.