It generally results from breaking a City Ordinance, such as illegal parking, letting your dog off a leash in a leash required area, or a very minor shoplifting offense.
Petty Misdemeanors, in terms of severity, are below Misdemeanors, which are below crimes. Petty Misdemeanors, by law, cannot require jail time and any fines imposed shall not exceed $300.
Ordinances are passed by cities, since cities cannot technically enact laws. Many ordinances are total bullshit. For example, in Chicago, 'exceedingly hideous people may not appear in public.' Or in Hillsboro, Oregon, 'your horse cannot ride in the back seat of your car.'
See Coaster Ordinance Loony Law
Mike: "Only a single candy bar, who stops someone for that?! What a douchebag!"
Jason: "Yeah, but it's probably just a petty misdemeanor"
David of Chicago: "Woah, check out that fuck ugly woman over there."
Pat: "I hope the cops give her a ticket for being so fuck ugly, she shouldn't be showing her face to the public."