What is the difference between the internal energy of an ideal gas and the internal energy of a real gas?

In any gas, particles (molecules) are constantly moving and hitting each other.
In an ideal gas, we neglect the sizes of molecules (we take them for material points) and consider all collisions to be elastic, neglecting their interaction. Therefore, the internal energy of an ideal gas is equal only to the sum of the kinetic energies of the molecules.
Due to collisions in real gas, the sum of all kinetic energies will not equal the internal energy of the gas.