A fixed-wing airplane with two sets of wings, one above the other. The Wright Brothers' airplane was the very first biplane.
Biplanes had lighter wing structures than early monowing planes, but they also had a lot more drag, and they needed a lot of bracing on the wings, which created even more drag. By the 1930s, improvements in structure and materials -- and higher speeds -- made the biplane obsolete.
Though very common in the early days of aeronautics, they are only flown today by aviation hobbyists or during exhibitions or air shows.