Freezing is the process in which a liquid gets converted to its solid state, on cooling. For example, liquid water when cooled down, undergoes freezing to become ice.
Boiling is the process in which a liquid get converted to its gaseous state, on heating. For example, liquid water boils and changes to steam, when it is heated for sometime.
Melting is the process in which a solid gets converted to its liquid state, when heated. For example, ice melts and changes to liquid water, when we heat it.
Condensation is the process in which a gas, when cooled down, gets converted to its liquid state. For example, steam undergoes condensation and changes to liquid water when it is cooled down.