Diatonic refers to a harp that commonly has 10 holes and plays the scale to which it is tuned. Diatonic Harmonicas do not have the button on the side. Making it the number one choice for professional harp players who want to play Blues, Folk, Country, Rock, Reggae, and Funk.
Chromatic harmonicas do have the button on the side and that enables them to play all 12 keys. They are considerably more difficult to master.
If you have a Chromatic Harmonica it will make it challenging to use the majority of techniques in my lessons. However, you could first learn the diatonic, and then move to the chromatic. Many of the techniques you will learn on the diatonic will also apply to the chromatic harmonica. Also, diatonic harmonicas are generally MUCH cheaper than chromatic harmonicas. So, I think you should spend some time learning the diatonic harmonica, and then you could switch to chromatic harmonicas if you feel the need.
Diatonic players generally find it easy to play chromatic harmonicas, but it's usually not that easy the other way round!
You'll need a 10 hole diatonic Harmonica to follow along with my lessons. The Hohner BluesBand is included in my "Complete Beginner Bundle" package for this purpose.
After you follow my techniques, you'll be able to adapt your playing to other harmonicas (like the chromatic) but it is recommended that you start with the included diatonic Hohner BluesBand for learning the building blocks.