The most common type of harmonica, also sometimes known as the Richter harmonica after the person who supposedly invented it in 1825. These instruments are often generically referred to as blues harps.  The diatonic harmonica is a single voice instrument and usually has 10 channels, each with one blow and one draw note.

Because these harmonicas are tuned to a single key (ignoring the possibility of playing in second/third/etc positions), most players will require more than one in order to be able to play a variety of songs with other musicians. But fortunately they are relatively inexpensive compared to their chromatic counterparts.