When heat is supplied to a solid, it expands. This expansion is very small. In fact, after receiving thermal energy, particles (atom or molecules) vibrate more rapidly in their position and take up more space. If particles become more energetic on further heating they leave their fixed positions and the solid melts.
Once a solid becomes liquid it can be poured into a container. A liquid takes the shape of the container in which it is poured. Particles in the liquid state are free to move.
When a liquid is heated, on receiving heat (thermal energy) a liquid is converted into a gas. This happens because the kinetic energy of the particles becomes so high that they can overcome the inter-molecular force within the liquid. Therefore liquid is converted into gas (vapour).
When a gas is heated, kinetic energy of the particles increases. They move more freely and at much higher speed. Inter-molecular distance also increases and the volume of the gas increases if pressure is kept constant.
A pure solid turns to liquid at a fixed temperature or in other words conversion of pure substance from solid to liquid takes place at one particular temperature. This particular temperature is called melting point of that particular solid substance.
Similarly when the liquid cools down, it converts into solid at a particular temperature. This temperature is called freezing point of that particular liquid substance. The temperature at which a liquid boils and is converted into a gas is boiling point of the liquid.