MCBs or Miniature Circuit Breakers are electromechanical devices which protect an electrical circuit from an overcurrent. The overcurrent, in an electrical circuit, may result from short circuit, overload or faulty design. An MCB is a better alternative to a Fuse since it does not require replacement once an overload is detected. Unlike fuse, an MCB can be easily reset and thus offers improved operational safety and greater convenience without incurring large operating cost. 
Mcb's can be fit in normal CFL holders also. as CFLs give out light they also give out heat which increases your electricity usage amount so MCBs can be placed instead of CFLs. MCB stands for miniature circuit breaker.
a fuse occurs when the filament of the bulb breaks .

hope dis helps u out in any which way:)