Overcurrent is basically what it says, any condition which causes more than the design current to flow in the circuit, this can be caused by
overload or a
fault condition
Faults can either be:
A) Phase ->Neutral or Phase ->Phase (termed a short circuit)
B) One or many phases to earth (termed an earth fault)
An MCB is a minuture circuit breaker as defined in BS3871, BSEN60898 does not refer to them as such; prefering to apply the name 'circuit breaker', but the term MCB is still widely used as it makes it clear you are taking about devices to fit in a final distrubution board
MCCB is as you quite rightly point out is 'moulded case circuit breaker' and are the devices to BSEN 60947-2 used in panel boards...
Both MCBS (or CBs if you want to be technically correct
and MCCBs can protect against all the above types of fault if the design is carried out to make use of their characteristics.
Earth leakage shunt trip units are available for MCCBS, but the situations that require them are not that common, larger supplies are most often TN systems, and if Ze is a little high, then careful design of the installation should allow the avoidance of earth leakage protection at this level of distribution