Two properties distinguish an ideal gas from a non ideal one.
They are as follows,
1) The molecules of a real gas occupy a finite volume while ideal gas molecules have a negligible volume.
2) There is no intermolecular forces in an ideal gas apart from the elastic collisions between the molecules. However, intermolecular forces do exist in a real gas.
As a consequence of the second point, another remark can be made that the internal energy of an ideal gas only depends on the temperature according to the kinetic interpretation of temperature. However, for a real gas, potential energies also come to play and the internal energy is no longer a function of temperature alone, it depends on volume and pressure as well.