Coil Voltage Rating of Relay
Some are 5V some are 12V and so on.. . What I want to know is, why there's difference is in this value?


Relays are made with different coil voltages to match the voltage used in the circuit that is controlling the relay.


  Do it have something to do with the force with which switching contact are touched and released?


Indirectly. The coil has to exert enough force to move the contacts against the spring that returns them to the un-energized position. If the voltage is too low then the magnetic field and therefore the force is too weak. If the voltage is too high then power is wasted in the coil and it might overheat.

Different coils are made with different sizes of wire and numbers of turns around the core, such that the right strength of magnetic field is created when the specified voltage is applied. For more information about this, look up the design of solenoid coils — 'solenoid' here refers to the simple cylindrical shape of the coil rather than the electromechanical plunger device.