microstates and internal energy Consider a system having an internal energy $U$. The internal  energy $U$ is a macrostate parameter but has many microstates. What is the difference between 2 microstates for a given internal energy $U$?
 A: Easy metaphor
Imagine that you have a pile of coins. Put them all in a line and flip them. The number of heads that you get is a macrostate. Which coins are showing heads is a micro state.
That is
HHHHHTTTTT

and
HTHTHTHTHT

are both states with 5 heads and so represent the same macrostate, but there are different microstates. 
There happen to be
$$ {10 \choose 5} = 252 $$
different ways to get 5 heads on ten flips, meaning there are 252 microstates that all correspond to the same macrostate.
A hand-wavy physical system
Take an ideal gas. The total internal energy is simply the sum of the kinetic energy of all the atoms. You can accomplish that macrostate (internal energy $U$) in different ways by giving different speeds to individual atoms (microstates). For physical version of this system there are very, very large numbers of different ways to arrange that energy.
A: An analogy:
Consider a bunch particles in an Harmonic oscillator potential;
At a given time, if you know the total energy of the system (the particles as a whole), then this is the macro state information; however, knowing the total energy does not say anything about the energy of the individual particles.
For example, it the total energy is say 5, and we have 3 particle, the energy distributions can be one of the followings:
$U_1 = 1, U_2 = 3, U_3 1$
$U_1=2, U_2 = 2, U_3= 1$
and any permutation of those two cases if the particles are distinguishable. Therefore, if they are indistinguishable particles, we have a total of 2 micro states for the given macro state $U_t = 5$.
So what is the difference between them ?
Basically nothing; but that is not the point why we are defining them because we use them to do statistical analysis about the system with the assumption that the probability of system being in the each micro state is equal, which leads to the result that the more there are micro states of a given macro state, the more likely that the system is in that macro state.
