Functional Definition:

Colligative properties are characteristics of solutions that depend only on the number of particles of solvent and solute present and not on the molecules themselves. Colligative properties include decrease in vapor pressure, decreasing melting point, increasing boiling point and changes in osmotic pressure.

Vapor Pressure: Vapor pressure in the case of a solution relates to the movement of surface molecules evaporating and condensing over time. At equilibrium the number of molecules that have evaporated and are condensing are equal and thus vapor pressure is constant. When a solute is added it decreases vapor pressure by adding molecules of solute to the surface and decreasing the evaporation and condensation of solvent molecules.

Melting Point: The addition of a solute decreases the melting point of a solution. By adding a solute you change the particle relationship of the solution by adding more molecules. An increase in molecules makes a solution more concentrated and lowers its ability to transfer energy; making it harder to order/organize the molecules. If it’s hard for a substitution to order its molecules then the solution is more likely to remain in a liquid disordered state.

Boiling Point: As with melting point you can imagine that the addition of molecules to a solvent makes the transfer/uptake of energy very difficult. The addition of a solute increases the boiling point of a solution because there are more particles that must be effected by the increase in temperature; more particles equals more heat. Just as it is difficult to order molecules in a condensed solution it is also hard to make them move or vibrate.

Osmotic Pressure: Osmotic pressure is the pressure needed to keep a pure solvent from passing into a solution by osmosis. Osmosis relies on the number of solvent particles in relationship to a solvent or another solution and the membrane barrier. In cells and other biological  systems the pressure created by solvent movement maintains a balanced osmotic pressure and prevents bursting or loss of necessary ions.