Trend: The general direction in which something tends to move.
A fad is fleeting. Fads are generally here today gone tomorrow like hypercolour t-shirts, legwarmers, tamagotchi's and probably foam (all the posh restaurants were serving flavoured foam at some stage in 2008).
A trend has staying power and generally lasts the decade it was concieved in. Ipod's are a trend as most people now own one and they have evolved into other industries such as sportswear, fitness centres etc. Businesses can generally make a lot of money if they recognise a trend over a fad.