"a fad" is something that is fashionable, normally for a short period of time, but that often doesn't really have much value (according to the person speaking). So for example "These new android and i-phone watches are just a fad, in five years time nobody will be using them!"
"a trend" is also something that is fashionable, but normally takes place over a much longer time period. It can either be something positive or negative (unlike 'a fad' which always has little worth). Also 'trend' is a more formal word, and so you are more likely to find this word used in scholarly or business articles.