There are really 3 main types of harmonicas:
1.Diatonic harmonicas are mostly used in Folk, Blues, Country, and Rock. They come tuned in a specific key and are easy to play in 2-3 different keys but it gets very technical if you want to play in all 12 keys with a single diatonic harmonica. You can get extra notes on a diatonic by bending and overbending.
2.Chromatic harmonicas used in Jazz, classical, and Pop music. This harmonica has a slider that gives you all the sharp and flat notes...