The difference between a leaflet and a flyer is the material it is printed on.
A leaflet is generally printed on to paper or light card and generally has a bit more information on it. A flyer is usually printed on thick card, somewhere around 300gsm (although I am now seen other companies printing them on a lightweight stock as well). Back in the day when I first started printing (back in the late 1980’s), we did not have flyers. They are a more modern product which are fantastic for short message regarding promotion or offers. The most popular size of a flyer would be A6 (which is a postcard size) and like I say are great for handing out to help promote an upcoming event or offer.