Ensemble and its Types

Ensemble 

 For any given macrostate (N, V, E), a statistical system, at any time t, is equally likely to be in any one of the extremely large number of its distinct microstates. With the passage of time the system switches itself from one microstate to another. So at a single instant of time we can consider the number of systems, i.e all being some sort of mental copies of the given system. For example we have a system of macrostate (N, V, E), having 'n' microstates. The system will switch itself from one microstate to another. So at a single instant the system can be in any of 'n' microstates. Thus we assume that there are 'n' mental copies of system which are present in 'n' microstates.
Ensemble are of mainly of three types :-

1)Microcanonical Ensemble
2)Canonical Ensemble 
3)Grand canonical Ensemble 

Microcanonical Ensemble :-

In this ensemble the macrostate of a system is defined by the number of molecules N, the volume V, the energy E. In this ensemble the range of the energy is specified, it is not sharply defined value but may lie in some range of energy.

Canonical Ensemble :-

In this ensemble the macrostate of the system is defined through the parameters N, V, T (temperature). As Practically a system can never have a fixed enegy or even an energy range. The total energy E of the system is hardly ever measured in direct manner.

Grand Canonical Ensemble :-

It comes from realization that not only the energy of the system but the number of particles in the system as well as is hardly ever measured in a direct manner. We can only estimate it through an indirect probing into system. So In grand canonical ensemble N and E are consider as variable and given by their expectation values <N> and <E>.