Quick answer
An RCD protects multiple circuits together. An RCBO combines RCD and circuit breaker protection in one device for a single circuit.
An RCD (Residual Current Device) is the big switch in your consumer unit that protects a group of circuits — when one circuit faults, the RCD trips and cuts power to all circuits it protects. An RCBO (Residual Current Breaker with Overcurrent protection) does the same job but for a single circuit only. If that circuit faults, only that one trips. RCBOs are better because a fault on one circuit does not affect the rest of your home. Modern consumer units increasingly use RCBOs for every circuit.
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