S-boxes substitute or transform input bits into output bits. A good S-box will have the property that changing one input bit will change about half of the output bits. It will also have the property that each output bit will depend on every input bit. P-boxes permute or rearrange bits across S-box inputs. In addition, at each round the key is combined using some group operation, typically XOR.
- Jonathan Katz and Yehuda Lindell, "Introduction to Modern Cryptography". CRC Press, 2007.