just curious how these two words are used in different situations...somebody already tackled this topic at wisegeek. This is my interpretation.
A fad refers to a fashion that becomes popular in a culture relatively quickly, but loses popularity dramatically. Some examples are: beanie babies (1990s), pet rocks (1970s), Rubik's cubes (1980s), and Pokemon (1998-2000). Fads tend to be short-term. For more fads, check out http://www.crazyfads.com/
A trend is a general direction in which something tends to move. Trends carry out over a longer period of time and have a greater influence on the future of a given market. This is what planners need to pay attention to in long-term brand development planning.
Fads seem more gimmicky -more product (not lifestyle/mindset) based. We can take advantage of these to gain and peak current interest, but need to watch trends to see where we can go with a particular movement of message.
just found flip.com and this search-worthy archive for "what is a trend at your school."
Go frolic and play, it's the weekend!