How do the three states of water differ?

The three forms (solid, liquid and vapour) are interchangeable and can change from one physical state to another. Chemically, they exist as H2O molecules throughout.

Solid ice melts on heating to become liquid water. Liquid water boils on heating to become gaseous steam.

Conversely, gaseous steam condenses on cooling to become liquid water. Liquid water solidifies on cooling to become solid ice.

On heating: ice -> water -> steam

On cooling: steam -> water -> ice