Air and 13 in Pro have only an integrated graphics card. Integrated means that the graphics chip is on the CPU, and has little memory. When the graphics chip heats up, the CPU can slow down to try to disperse heat.
15 and 17 in Pro have discrete and integrated graphics. That means there is also a graphics card that is NOT integrated into the CPU and has more memory than integrated chips. Becuse it is not in the CPU the heat from the graphics card does not directly affect the CPU. And you can do much more with pictures.
13/15/17 Pro you can upgrade the RAM and HD. Air you can only replace the SSD because the RAM is soldered to the logic board.
If you do much CAD, discrete graphics is better in my opinion. But 15 in Pro does start at around $1800 instead of around $1000.