A flyer is a single sheet of unfolded paper or card. A5 and A6 are the most popular. You can print on one or both sides. Furthermore, a leaflet will be folded. For example, A4 folded to 4pp A5 is an A4 sheet folded in half to provide 4 sides. The easiest way to remember what's what is by referring to them as flyers and folded leaflets.