There are several distinct types of concertina, all sharing the same basic design of folding bellows with buttons at each end, and anything from 6 to 12 sides in cross-section. Where they vary is in the layout and function of the keys.
Concertinas come in various sizes which govern the range of notes they can play.
The most common are treble concertinas. Below them are baritone concertinas which play one full octave below the treble, and the bass which plays one octave lower again. Also fairly common are tenor-trebles which cross the range of the treble and baritone. VERY occasionally you find piccolo concertinas which play one octave above the treble. The main types are the English, the Anglo and the various systems of Duet concertina.