All 7th gen core i5 are quad cores. All 8th generation core i5 are hexa-core (six cores).
More cache to go along with the increased core count. Slight architectural speed improvements, higher clock speeds.
They also use different chipsets and motherboards. H110, B150, B250, Z170, Z270 for 7th gen, Z370, H370, H310, and B360 for 8th generation.
Do not let the same LGA1151 socket trick you, 7th/8th gen are not cross compatible. (6th and 7th gen are though)