The popular types of music are unusually played on a 10 hole diatonic harmonica. This is the instrument most people think of when you mention a harmonica, and this is the type most of us had as kids. These models include some of the most popular harmonicas and are used in Blues, Folk, Country, and Rock.
All feature reeds which are tuned to produce the natural notes of the scale without any additional sharps and flats. These musical styles are usually played in “Cross Harp”, using riffs and fills that add to and complement the music rather than playing the song melody.
Even though you may be playing Rock or Country harmonica, the playing style used is very similar to playing Blues harmonica, indeed these musical styles themselves have their roots in the blues.
Diatonic harmonicas come in 12 standard keys for the (G, Ab, A, Bb, B, C , Db, D, Eb, E, F, F#) so you normally would change harmonicas to change song keys. There are some specialized techniques used by some players to play a diatonic harmonica chromatically, but most people start playing diatonically and work on those other techniques later (see Bending, Overblows and Valving a Diatonic for more information on these techniques.)