Winston Churchill played many roles in his long public life. However, for historical purposes, he is important in two main ways. These ways have to do with his actions during World War II and his role in the Cold War.
Churchill was in the public eye to at least some extent long before World War II. This started in the early 1900s when he was both a war correspondent and then a soldier in the Boer War in South Africa. He gained some degree of fame for his actions in those capacities. Churchill was First Lord of the Admiralty by the start of World War I. While serving in that position, he was instrumental in modernizing the British navy. He was also, on a less happy note, responsible for the British debacle at Gallipoli in that war.
However, it was during WWII that Churchill truly became prominent. He was an opponent of appeasing Hitler before the war. Early in the war (May of 1940) he became Prime Minister and continued in that office throughout the war. He was seen as instrumental in keeping up British morale and in directing the British war effort. His speeches during the hard times early in the war are seen as important factors that allowed the British people to maintain their confidence.
After WWII, Churchill was out of office for a time. However, he was still influential. In 1946, Churchill gave his “Iron Curtain” speech in the US. This speech helped to push the US towards a more confrontational attitude towards the USSR in the Cold War.
Thus, Churchill played two major roles in history. He led Britain through WWII and he helped to encourage the US to oppose the USSR strongly in the Cold War.