The Core i5-8400 brings a powerful six-core design to the mid-range, offering class-leading gaming performance and competitive performance in heavier applications. More expensive models offer more performance in both categories, but the Core i5-8400 is easily the pound-for-pound gaming champion.
Read more @ Tom’s Hardware
All these new processors are desktop parts, meaning they are socketed processors for use in appropriate motherboards featuring the Z370 chipset. Technically these processors use the LGA1151 socket, which is also used by the 6th Generation and 7th Generation processors with the Z170 and Z270 chipsets. However due to differences in the pin-layout of these two sets of processors, 8th Gen will only work in Z370 boards and there is no level of cross compatibility. We will discuss this later.
Read more @ AnandTech