What is the difference between an MCB and an RCBO?

An MCB (Miniature Circuit Breaker) is the standard switch in your fuse box that cuts power if a circuit is overloaded or short-circuits — like if you plug in too many appliances at once or there's a fault in a cable. It won't protect you from an electric shock. An RCBO (Residual Current Breaker with Overload) does everything an MCB does, but also detects tiny amounts of electricity leaking to earth — the kind that can cause a fatal shock. If you get an electric shock or a fault develops in a cable, an RCBO can cut the power in milliseconds. Modern consumer units ideally use RCBOs on every circuit instead of MCBs, because it means if one circuit trips, only that circuit goes off — not a whole group. An RCBO is basically an MCB and RCD combined into one device.