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The first three positions cover the vast majority of harmonica playing, and these positions have names as well as numbers. They are:
1st position (or "straight harp"): Ionian mode. Playing the harmonica as it was intended, in its main major key. On a diatonic, starting note is hole 1 blow. On a C-chromatic, starting hole is the same, resulting in C major scale. This is the main position used for playing folk music on the harmonica.
2nd position (or "cross harp"): Mixolydian mode. Playing the harmonica in a key a fifth above its intended key. Playing just the unbended notes, this position gives the mixolydian scale between 2 draw and 6 blow. However, bending the 3 draw allows the player to play a minor third (or a blue third), allowing a player to use a C harmonica to play in G mixolydian or G minor. Blues players can also play a tritone in this position by bending the 4 draw. See a more extensive discussion of this position at the article on blues harp. On a diatonic, starting note is hole 2 draw or hole 3 blow. On a C-chromatic, starting hole is hole 3 blow, resulting in G major with a flatted 7th.
3rd position (or "double cross harp" or "slant harp"): Dorian mode. Playing the harmonica a full tone above its intended key. This gives a dorian scale between 4 draw and 8 draw, though once again bends and overblows give players a variety of options. Blues players can achieve a tritone by bending the 6 draw. On a diatonic, starting hole is hole 1 draw. On a C-chromatic, starting hole is hole 1 draw, resulting in D-minor with a raised 6th. This is the traditional way of playing Blues on Chromatic.
The terminology for other positions is slightly more varied. It is possible, of course, to play in any of the modes and, using overblows and bends, it is possible to play in all 12 keys on a single diatonic harmonica, though this is very rarely done on a diatonic. Even if a player is capable of playing multiple keys on a single harmonica, they will usually switch harmonicas for different songs, choosing the right "position" for the right song so as to achieve the best sound. Even when the same notes can be achieved on different harmonicas, choosing the different harmonicas will offer a variety of options as to slides, bends, and breathing patterns which will allow the player to create a different sound overall.
