The K suffix denotes that the processor is an unlocked variant and ca be overclocked due to the unlocked multiplier. Processor of this range don't need to undergo BCLK overclocking. The i5-6600K is also rated at 91W TDP as opposed to the non K variant which is 65W so less heat output for the non K and on that subject, the K variant doesn't come with a stock cooler since logically Intel figured enthusiasts will buy an aftermarket cooler anyway for overclocking.