A fad is a fashion or activity/behavior that is fleetingly popular. While many people may use the terms fad and trend interchangeably, a fad is often less permanent than a trend, the latter of which usually has more staying power. There are plenty of examples of fads. A big fad a few years ago was Uggs shoes. Additionally, other fads over the years have included but not been limited to leg warmers, headbands, and scrunchies.
A true style master recognizes fads, and ignores them at his pleasure. He can also leverage this cultural mass group psychology to take advantage of it. For example, dressing up like a recent famous rocker who had bad press as a joke, or making fun of an existing fad by wearing it in correctly.
The difference between fad and passe is that passe was once recognized as consistently fashionable, and run out in due time. A fad can change as quickly from day to day.