The word flute is a broad term covering many instruments, from the modern orchestral woodwind to folk instruments of most cultures. The modern orchestral flute has 13 main tone holes, other smaller holes to facilitate trills and other fingerings, and elaborate keywork. The control of the sound is achieved by the player’s lips; a proper embouchure is a crucial part of the instrument’s technique.
The flute family includes the piccolo, half the size of the concert flute, the alto flute and the bass flute (used very rarely), an octave lower than the concert flute.
Flutes of various sorts were known in ancient civilizations. After 1500, flutes appear in pictorial and literary sources throughout western Europe. In the early Baroque, the flute fell into decline; the inefficient Renaissance design could not meet the music’s demands. In the Classical era, with improvements in its design, the advent of public concerts, the growth in amateur music-making and the rise of the expressive galant style well suited to it, the flute greatly gained popularity. The flute entered the orchestra in the mid- to late-18th century. Often flutes were an alternative to oboes. In the 19th century two flutes became standard, and sometimes a piccolo was included. In music of the 20th century, the alto flute makes an occasional appearance.
Source: The Norton/Grove Concise Encyclopedia of Music, edited by Stanley Sadie. London: Macmillan Press Ltd.,  ISBN 0-333-43236-3