RCCB: Stands for Residual Current Circuit Breaker. It is also known as Residual Current Device (RCD). RCCB senses the difference between current values in phase and neutral and breaks the circuit in case of an imbalance. In ideal situations, the current flowing through phase (hot) wire equals the returning current in the neutral. However, in case of fault conditions such as an earth fault the current flowing through neutral would not be same as flowing through phase and hence it trips the circuit.
ELCB: Stands for an Earth Leakage Circuit Breaker, is used to protect the circuit from an earth fault. It is an old protection system based on voltage based protection device. It is nowadays replaced by RCCB.
ELCB is connected between the circuit and the system earthing system. It is a voltage operated device and detects any rise in potential between the circuit and the system earth and trips the circuit.
RCBO: Residual Circuit Breaker with Overload. Is a circuit breaker that offers both Residual Current and Overload protection in one single device.
GFCI: Stands for Ground Fault Circuit Interrupter. It is another name for RCCB/RCD. This term is used in USA whereas RCCB/RCD term is used in UK and Europe. Functioning of both is much the same.