[Bakhtiyar]

In ideal gas , we assume that particulars are so small and they have no interaction between particulars until the collision with wall. Ideal gas with the conditions (chemically nonreactivity and always being in equilibrium) is Perfect gas .

[Arkcon]

I would start here:  http://en.wikipedia.org/wiki/Perfect_gas  and also check the links on that page to Ideal gas.  Then, see what you still don't understand, and ask about that.