A transverse flute is an instrument that you hold sideways or horizontally to play. You create sound by directing a stream of air toward the sharp edge of an opening (in the mouthpiece); different pitches are created by blocking groups of holes along the body of the flute with your fingers. These instruments can be made from metal, wood or bone or even glass.
Before the mid-19th century, western flutes only had open holes which a player covered with their fingers to create different tones. However during the 1830s, Theobald Boehm developed a system of keys and springs to replace many of the open holes. The Boehm invention required a complete fingering change for the flute and so was slow in gaining popularity, but today the Boehm flute is used almost exclusively in contemporary western orchestras.