Diatonic Harmonica Defined- The standard 10-hole diatonic harmonicas come in all 12 keys of music and allow you to play a complete 7-note scale of the key of the harmonica. Additional notes from outside the scale can be acquired by "bending" certain draw (inhale) and blow (exhale) notes. Most professionals primarily play diatonic harmonica. Diatonic harmonicas are typically used in blues, rock, country, and folk, but can also be found in nearly all styles of music. They are sometimes referred to as a "blues harp", "harp", "short harp", or "standard 10-hole".
The particular model and brand of diatonic played by your favorite pro is not important when choosing a harmonica, more importantly, learn to play like the pro and you will sound like them. If you are at all serious about learning to play, start with a good quality one. The cheaper ones leak air, are very difficult to learn to bend, and can cause no end of frustration.
Note: All keys of standard diatonic harmonicas are layed out and played the same way. That is, if you own a standard 10-hole diatonic in a key other than the key of "C" (like an "A" or "D" for instance), it will play the same as the "C" for songs, riffs, and instruction, but the music comes out in a different key.