The tin whistle, or flageolet, or pennywhistle, or Irish whistle, or simply whistle, is a simple six-holed breath instrument. It can be described as an end blown fipple flute, putting it in the same category as the recorder, Native American flutes, and many other wind instruments found in traditional music.
The most common tin whistles today are made of brass tubing, or nickel plated brass tubing, with a plastic mouthpiece (Generation, Feadog, Oak, Acorn, Soodlum's, and other brands fall in this category). The next most common form is the conical sheet metal whistle with a wooden stop in the wide end to form the mouthpiece, the Clarke's brand being the most prevalent. Other less common variants are the all-metal whistle, and the wooden whistle. Whistles are a prevalent starting instrument in Irish traditional music, since they are cheap (under US$10), relatively easy to start with (no tricky embouchure such as found with the flute), and the fingerings are identical to those on the traditional six holed flute (Irish flute, baroque flute).
Whistles may or may not be tuneable. If they are, tuning is done by moving the mouthpiece in or out, either the mouthpiece itself sliding over the whistle body, as in the metal tube/plastic body model, or else with a tuning slide such that the mouthpiece and the upper part of the body form the 'head' of the whistle which fits into the main body. Some whistles allow one head to be used on differently keyed bodies this way.
The whistle is in a single key, usually D for Celtic music, and C for American folk music, though whistles are available in all the major keys. The notes are generated by opening or closing holes with the fingers, such that with all the holes closed the whistle generates the tonic, with the lowest hole open it generates the second, and so on. Once the seventh is reached, the next note is achieved by closing all the holes again and blowing more forcefully to achieve the octave. In this way, one can go up two full octaves. The standard range of the whistle is from the D above middle C, to the D two octaves above middle C. (It is possible to make noises above this range, by blowing increasingly forcefully, but it is not recommended, both because the sound will be off pitch, and because it may result in an aggravated assault by nearby listeners...)
Although the whistle is essentially a diatonic instrument, it is possible to get notes outside the principal major scale of the whistle, either by half-holing (partially covering the lowest finger hole) or by cross-fingering (covering some holes while leaving some higher ones open). In this way, for example, a whistle pitched in D can easily be made to play in G (or Em), and other keys are available with a little more work.
There are larger whistles, which by virtue of being twice as long are an octave lower; whistles in this category are likely to be made of metal or plastic tubing, with a tuning-slide head, and are almost always referred to as low whistles. 'Chieftain' is a famous maker of low whistles. The low whistle operates on identical principles to the standard whistles, but musicians in the tradition may consider it a separate instrument.