Unlike most free-reed instruments (e.g., reed organs, accordions and melodicas), the mouth harmonica lacks a keyboard. Instead, lips and tongue are used to select one or a few of the several holes arranged usually linearly on a mouthpiece. Each hole communicates with but one, two or a few reeds. Because a reed mounted above slot is made to vibrate more easily by air from above, reeds accessed by a mouthpiece hole often may be selected further by choice of breath direction (blowing, drawing).
Some harmonicas also include a button-actuated slide that, when depressed, further redirects the air.
See Pan pipes.
The harmonica consists of a "comb" made of wood or plastic which creates the holes into which a player blows or draws to make distinct tones. The metallic blow and draw reedplates are screwed onto either side of the comb. Over the reedplates, there is a metal or plastic cover which projects the sound out of the open back. Chromatic harmonicas also have a button-activated slide.
The diatonic harmonica is most likely what you think of when you think of a "harmonica." It has ten holes which offer the player 19 notes (10 holes times a draw and a blow for each hole minus one repeated note) in a three octave range. The standard diatonic harmonica is designed to allow a player to play chords and melody in a single key. Because they are only designed to be played in a single key at a time, diatonic harmonicas are available in all keys. Here is a standard diatonic harmonica's layout in the key of C:
1 2 3 4 5 6 7 8 9 10 ------------------- blow: |C|E|G|C|E|G|C|E|G|C| draw: |D|G|B|D|F|A|B|D|F|A| -------------------
Note that there is only one full major scale available on the harmonica, between holes 4 and 7. The lower holes are designed around the tonic (C major) and dominant (G major) chords, allowing a player to play chords underneath a melody by blocking or unblocking the lower holes with the tongue.
In addition to the 19 notes readily available on the harmonica, players can play other notes by adjusting their embouchure and forcing the reed to resonate at a different pitch. This technique is typically called "bending" and allows to a player all the notes on the scale as well as pitches in between. "Bending" creates the glissandos characteristic of much blues and country harmonica playing. The physics of bending are quite complex, but amount to this: a player can bend a note down toward the pitch of the lower-tuned reed in that hole. In other words, on holes 1 through 6, the draw notes can be bent and on holes 7 through 10 the blow notes can be bent.
Howard Levy[?] developed another technique in the 1970s that allows players to force a reed to vibrate faster, resulting in a higher pitch. This technique is called overblowing or overdrawing and is much less frequently used. For the few who master this technique, the diatonic harmonica can function as a fully chromatic instrument.
List Modern Overblow Masters:
The chromatic harmonica has a button-operated slide that allows the player to change the pitch of any given hole. This means that each hole has 4 pitches rather than 2. The slide typically shifts the pitch of any given note by a half step. The note layout on a chromatic is traditionally the same as the note layout on holes 4-7 of the diatonic harmonica, and is repeated over its length. Chromatic harmonicas are usually 12 or 16 holes long.
Because it is a fully chromatic instrument, the chromatic harmonica is the instrument of choice in jazz and classical music. In traditional harmonica bands, the chromatic harmonica plays the lead part.
Please add some text if you know something of the bass harmonica. Lengthy description at: http://www.bassharp.com/bh_101.htm
Please add some text here if you know something of the chord harmonica.
Echo harmonicas have two reeds per note, one of the reeds slightly out of tune. This produces a tremolo effect.
Please add some text here if you know something about the echo harmonica.
Please add some text here if you know something about the toy harmonica.
The unrelated glass harmonica[?] is a musical instrument formed of a nested set of graduated glass cups mounted sideways on an axle and partially immersed in water, and played by touching the rotating cups with wetted fingers, causing them to vibrate.
There is an active harmonica community on the internet and in real life, with conferences, cruises and everything. SPAH (Society for the Preservation and Advancement of the Harmonica) is one society with a particularly amusing acronym.  (http://members.aol.com/harmonica) A harmonica list-serv is hosted at this web site (http://www.garply.com/harp-l/) with searchable archives.