A circuit breaker is designed to break a circuit which is carrying current at the time. Where large currents and/or voltages are present, the opening of the circuit may induce dangerous arcing, so means such as air blast must be used to "blow out" the arc. Air breakers are being phased out and replaced with vacuum or SF6 breakers.
An isolator is only designed to isolate a circuit or other component after the current has already ceased, so it doesn't need to have any means to stop the arc.
Thus in high voltage/high current installations such as in power stations or grid switching stations, the initial operation to isolate the circuit, whether planned or as a result of a fault, is by the circuit breaker, the isolator is then used to isolate the circuit breaker for maintenance or repair.
The difference between a circuit breaker and a contactor is that a circuit breaker can break fault current while a contactor cannot. A contactor is typically implemented in series with a fuse which serves to interrupt fault current. An isolator and a contactor are not synonymous.