Muhammed Ali was the adopted name of Cassius Clay (b. 1942). He is a retired American boxer and former three-time World Heavyweight Champion; possibly the most famous boxer of all time. Through his career Ali had 61 professional fights and went 56-5. Giving up his heavyweight title when he refused to go to fight in the Vietnam War, Ali lost important years in his prime. Coming back after he was allowed to return to boxing, Ali beat the heavily favored George Foreman in the famous "Rumble in the Jungle," a bout held in 1974 in Kinshasa. It was Ali's first use of the "Rope-A-Dope" strategy, where he covered up and allowed his opponent to tire himself out with useless punching. Ali's most famous line during his fighting days was, "I'll be floating like a butterfly and stinging like a bee."
Always undersized for a heavyweight, Ali made good use of his speed and technical boxing skills. His skills started to erode quickly once he reached a certain age and he lost his title to a lightly regarded Leon Spinks, who went on to become a footnote in boxing history. Ali took the title back in a lackluster decision, but retired soon thereafter. He tried to make a comeback against then champion Larry Holmes, but out of shape and past his best fighting years, Ali's corner threw in the towel between rounds in the middle of the fight.
After his boxing career, Ali was diagnosed with Parkinson's disease. Whether this was caused by his boxing is currently a subject of debate.
Ali was not without controversy in his career. In his rematch with Sonny Liston after taking the heavyweight title for the first time, Ali knocked out Liston in the first round with what has come to be called a "phantom punch". In other words no one is exactly sure what made Liston go down, but it doesn't appear to have been Ali.