When picking out your first harmonica it can be confusing to find which one to start with. There are two major types of harmonicas the Diatonic and the Chromatic.
The Diatonic is commonly called the blues harp and is the most common type of harmonica and is the one that most beginners will purchase. The diatonic harp gets it's name from the fact that each key harmonica that you have only has notes from the scale for that key. So, for the C harmonica, the notes on the harp are C, D, E, F, G, A and B. For A, it would be A, B, C#, D, E, F#, G#, etc. Each diatonic harmonica is designed for one key. Each set of keys is actually set when the harmonica is made. This means that as a harmonica player advances they will want to purchase additional harmonicas in different keys to play more songs.
The Chromatic harmonica has a button, which raises the pitch of the note by redirecting air from the hole in the mouthpiece to the selected reed-plate. Any note you play will be raised when the button is pushed, whether you are blowing or drawing on that particular hole. This allows for chromatic playing, of a full octave of 12 notes. So a Chromatic which is Richter-tuned will end up acting like two diatonic harmonicas combined into one. Some 12, 14 & 16 hole models which are tuned to equal emperment will enable the full reproduction of all the keys on one harmonica. Sometimes you will also hear the about a Tremolo harmonica. The Tremolo distinguishing feature is that is has two reeds per note with one sharp and the other flat. The combination of these two notes causes a warbling sound. For Recommendations of the best harmonicas for a Beginner click here.