circuit breaker is used to trip during fault condition of 
the circuit whereas isolator are the off load is used for 
isolate the circuit during maintenance and shutdown

Circuit breaker is device whcih disconnect the equipment 
during fault condition automatically,but isolator does not 
disconect the equipment automatically during fault 
condition.
Isolator should be operate after deenergizing the 
line,because isolator is not having any special arc 
quenching facility like sf6 or vaccum in it.cb is having 
this facility.
Note:- Isolator is a OFf load device it should follow the 
CB operation

there is no arc quenching medium in isolator .
also we have earth swith in isolator for disharge amd 
safety purpose

In the simplest form: 

An ISOLATOR is a manual circuit breaker while Circuit 
Breaker does this with the help of a relay.

Regarding the arc quenching medium, isolator too have air 
as arc quenching medium.

And,

An iso. is an off-load device but a CB is onload.

The basic difference between cicuit breaker & Isolator is, 
Circuit breaker ,automatically  breaks the circuit  
electrically in conditions of fault & overload, where as 
Isolator is to be switched OFF manually  after tripping the 
prevoius protection / drive for the purpose of 
maintenance/shutdown.

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. 

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 main difference between circuit breaker and isolater is 
circuit breaker can operate in any condition ie in full 
load to no load condition but isolater can operate in no 
load condition.
  circuit breaker can operate both manually and 
automatically but isolater can operate only manually.

cir cute breaker used to avoid accident in fault condition
its automatically off Isolater which used to manually off in
fault condition

Circuit Breaker is operated ON-LOAD whereas ISOLATOR can 
not be operated ON LOAD as it does not have the arc 
quenching medium as in the CIRCUIT BREAKER.
ISOLATOR is basically used for disconnecting a portion of 
the power system for maintenance work to ensure safety of 
persons doing maintenance work.