What is the difference between the iPhone 4, iPhone 4S and iPhone 4G?

Is there any difference is the size of tehm, because i wanted to buy a case buttm not sure if it will fit, because it says it fits the 4S, $g and iPhone 4 but i'm still not sure.

There's no "iPhone 4G". There's the iPhone 4 & iPhone 4S. Physically the phones are the same size but have minute changes. The volume buttons, silent switch & headphone port between the 4 & 4S are slightly repositioned. So you have to be specific when buying a case.

The iPhone 4 and iPhone 4s have similar designs, but the volume and silent buttons on the left side are slightly closer to the top meaning some cases won't fit on them. The iPhone 5, sometimes called the iPhone 4G, has a completely new design, and is taller, thinner and has relocated buttons and jacks