There are basically three types of harmonica, though a few more have come into existence with even more modified features.the right selection of the desired harmonica should be based on the style of music one wants to play and the kind of effects he wants to put in. There are 3 basic types: the diatonic harmonica, the tremolo harmonica and the chromatic harmonica.

1. Diatonic Harmonicas This was the first type of harmonica to be created. Although they are most commonly used to play blues and country, one will be able to play all kinds of popular music with a diatonic harmonica.
A diatonic harmonica has 10 holes and permits the player to use the tones and semi-tones equivalent to the white notes on a piano (using the scale of C major as an example) However, it is possible to play other notes by using bending and over blow techniques.

2. Tremolo Harmonicas: This type of harmonica is ideal for playing very traditional music because one may easily create a wonderful tremolo effect. It is very popular with Gospel and old world; Folk, Country, Scottish and Irish music.
Tremolo harmonicas are constructed with double holes placed vertically. Each contains two reeds, one slightly de-tuned to the other. When vibrating together, these reeds produce a unique tremolo effect.

3. Chromatic Harmonicas: The chromatic harmonica has been created to compensate for the missing notes on the diatonic harmonica. You will be able to play both the tones and semi-tones of a chromatic scale (a chromatic scale is when both white notes and black notes are played in succession; C, Db, D, Eb, E etc) helped by a slide button.

A few other types of harmonicas are-

OCTAVE TUNED-
Octave harmonicas are similar to Tremolo models in reed layout and musical range. Instead of having reeds tuned to the same note, however, each double hole has one reed tuned an octave apart from the other.

SPECIAL TUNED DIATONICS- This is used for enjoying the advantage of additional notes and scales which are not present in chromatic harmonica.

CHORD AND BRASS HARMONICA- These are two more types of harmonica which are basically used for ensemble performances.

There are thousands of different models present in the market,be it a diatonic or a chromatic harmonica,and different models of different other types of harmonica.harmonicas can be made up of metal or plastic.