A tin whistle is a simple instrument in the woodwind family. With just six finger holes, the tin whistle has a two-octave range and is tuned diatonically (i.e. containing all seven notes -- and no others -- of a major or minor scale). The tin whistle is a very common instrument in traditional Irish music and related genres of Celtic music.
Despite its relative simplicity as an instrument, the tin whistle, when played by a skilled player, can be a dynamic and exciting instrument with a surprising level of nuance. That said, it's a great instrument for a beginner because of the relative simplicity of both producing sounds (the mouthpiece is simple... you just blow!) and picking out melodies. Additionally, a factory-made concert-quality instrument can retail for under $20.