from wiki "When the button is not pressed, an altered diatonic major scale of the key of the harmonica is available, while depressing the button accesses the same scale a semi-tone higher in each hole. Thus, the instrument is capable of playing the 12 notes of the Western chromatic scale."

I have a 12 hole chromatic in the key of C. So when I don't push the button, my chromatic is a diatonic in the key of C? I just got it so I'm trying to get a grasp on things. I want to make sure the music I'm trying to play is correct for my harmonica but this music theory is confusing me @_@

It's kind of like a diatonic, like the middle octave of a diatonic. Notes are left out of the 1st and 3rd octave of the diatonic. It's called Richter tuning, the chromatic is called solo tuning, all three octaves are made the same way. Go to a site like coast to coast and find the layouts for the various types of harps. For a hint, all the notes between -3 and -8 on the diatonic can be played on the chromatic one hole higher. -3 is -4, -7 is -8, etc. Find the harp layouts and you'll see how this works. Of course you could just stick to my chromatic tabs and not have to worry. (That's a joke)
Good luck.