Straight from Wikipedia.
Censors initially refused to allow Fonzie to wear a leather jacket, thinking that it made him look too much like a hoodlum. Garry Marshall (creator of Happy Days) got them to compromise, agreeing to allow Fonzie to wear his leather jacket when in close proximity to his motorcycle (since a leather jacket is a legitimate piece of safety equipment). To ensure that Fonzie would wear the leather jacket at all times, Marshall instructed the writers to put him near his motorcycle as often as possible, even going so far as to have him ride it into Arnold's. Even so, for some first season episodes, he wears a white jacket. Eventually, the censors relented, and Fonzie was allowed to wear the leather jacket even when not near his bike.
Fonzie actually wore two leather jackets regularly - a black one and a dark brown one.
On an episode of Happy Days, Fonzie met Mork, a wacky alien. Played by Robin Williams, the Mork character proved so popular that he received his own spin-off series, Mork & Mindy.