What is the difference between MPCB and Over Load Relay?

A motor protection circuit breaker (MPCB) is used to protect from overload and/or short circuit currents. An overload relay is used for overload only, and as such will have a relatively long delay before tripping. Also, I believe some overload relays are a one-use device (once tripped, they must be replaced), and I don't believe this is the case with MPCBs (could be wrong though).

If you look at a manual for a MPCB, you'll see the tripping curve is two curves in one - at low currents, it is very slow (time in hours), but once you get above load currents, it speeds up significantly down to a few cycles, which is what you want for faults / short circuit conditions. An overload relay will have the first (slow) curve only.

Do a google search for Cutler - Hammer (or go to their website) for some examples.