An ideal gas is a theoretical gas composed of a set of randomly-moving, non-interacting point particles. The ideal gas concept is useful because it obeys the ideal gas law, a simplified equation of state, and is amenable to analysis under statistical mechanics.
A gas consisting of particles that could be atoms or molecules is called an ideal gas if the particles that constitute the gas collide with each other in a perfectly elastic manner. Also the particles should not have any attractive forces between each other. If the gas can be pictured as one that is made up of perfectly hard spheres with no inter particulate attraction it is called an ideal gas. Internal energy in an ideal gas is purely kinetic, and a change in temperature is accompanied by a proportional change in energy.
This can be expressed by the relation PV= nRT perfectly. Here P is the pressure, V is the volume, n is the number of moles, R is the universal gas constant and T is the temperature. Gases around us are not ideal gases, so the ideal gas law is a close fit but does not exactly express the properties of gases.