What is the difference between an accordion and harmonium?

The accordion and the harmonium are both free reed instruments because the reed used to produce sound are standing freely. However, the instruments are not played in the same way.

The accordion is also known as a squeeze box, the instrument is played by expanding and compressing the bellows and pressing the keys. The pallets open up and air enters the reed. The reeds vibrate to produce a sound.
The harmonium also called pump organ or melodeon produces sound when air enters the thin reed.