What's the difference between a real gas and an ideal gas?

Well the main difference between a real gas and an ideal gas is that gases that behave as the kinetic molecular theory predicts are refereed to as ideal gases.

Ideal gases are easier to work with because we can predict how they are going to react because they are under certain conditions.  In other words, if we were observing real gases, we would have to account for variables outside of the four variables that are used to describe ideal gases.
 

Although it is difficult to define what an ideal gas is because all gases do have mass and exert pressure on their containers, it is best to refer to the kinetic molecular theory to decide if the a gas is ideal.