"Trend" means a general development or change in a situation. "Fad" means a style, activity, or interest that is very popular for a short period of time.

"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.

A "trend" is a general change in some processes or social preference for something. It could also mean a pattern that can be inferred from some data points. Examples: There is definitely an upward trend in global temperatures over the last few decades. / Upstarts such as Uber and Airbnb are examples of a global trend towards a shared economy. / A fashion trend towards androgynous styles. A "fad" is something that generates enthusiasm and interest in a large group of people or someone but the craze is temporal and fizzes out quickly. Examples: Tetris was my childhood fad. / To me, Gangnam Style was a fad. In a nutshell, "fads" are short-lived interests in something and may not spring from any predecessor while "trends" are a shift from some previous condition or situation to a new one.