The one row melodeon only allows you to play in that one key that the instrument is tuned to. The Cajun musicians generally use a 1 row with 4 sets of reeds. They sometimes come with two bass levers or also with four bass buttons. A different note plays depending on whether the bellows is pushed or pulled. Two row models normally have 8 bass buttons.
Concertinas are small, usually hexagonal instruments.
Pictured left is an instrument with the English system. This has more buttons and plays the same note regardless of the direction of the bellows. Picture right is the Anglo-Chromatic model and this has fewer buttons but plays a different note when pushing or pulling the bellows. As a rule they are generally less expensive. The cheaper end of the market is the Chinese model and it starts at approximately 100.