The alto flute is a transposing instrument. This makes it easy for someone who plays the flute to play on the alto flute. The fingering for the note C on the flute will give a G on the alto flute.
The alto flute is not very often used, and only in very large orchestras. Maurice Ravel wrote a beautiful tune for alto flute in Daphnis et Chloé. Igor Stravinsky used one in The Rite of Spring. In The Planets by Gustav Holst there is a passage with three flutes and alto flute playing 4 part harmony.
The alto flute must not be confused with a bass flute which is even bigger and lower.
Media related to Alto flutes at Wikimedia Commons