The Kulaks were a group of the most prosperous peasants who refused to move off of their land and onto collective farms. Under Lenin, they had achieved some prosperity through limited privatization of property, which they did not want to give up. The Kulaks did not want this way of life and rebelled, prompting a huge crackdown by Stalin. The irony is that after stifling the Kulaks later Stalin approved of the limited privatization of property on collective farms.
When the Kulaks refused to move onto collective farms, Stalin began a crackdown on the Kulak people. Kulaks began to destroy their livestock and cattle. Millions of the Kulak were sent to Siberia or brutally murdered. Stalin would send troops to surround villages and simply machine-gun everyone in the village, including women and children. Because of this stifling of farms, agricultural production went down and famine reigned. Ukraine was the hardest-hit republic, and millions of Ukrainians starved to death. Stalin later admitted that through the Kulak massacres and the famine roughly 10 million people had died. Stalin killed a total of 20 million of his own countrymen when you add The Great Purge into consideration.