The answer is (provided the ELCB has been made within the last 20 years or so, ones produced before then may have been voltage operated and were often used in areas which had very poor ground earth resistance figures) nothing, zero difference its just a change in terminology. ELCB refers to an Earth Leakage Circuit Breaker. Any current leaking away to earth will cause an imbalance between the live and neutral wires that pass through the toroidal transformer and this will produce a small voltage on the detection coil wound around the toroidal core which will operate a sensitive relay to operate and cause the circuit breaker to trip, disconnecting the supply. This imbalance causes a current to flow in the detection winding, this is called Residual Current, so therefore a RCCB is just the old ELCB brought upto date with the new terminology and is called a Residual Current Circuit Breaker.
Does that help? If you require more info or some diagrams that show how these work, google is your friend.