Ivan Pavlov was a Russian physiologist who won the Nobel Prize in 1904 for his work on the physiology of digestion. He is best known for his classical conditioning experiments using dogs. Pavlov discovered that dogs could learn to associate stimuli with food through repeated pairings, demonstrating conditional reflexes. He also studied innate reflexes, which are instinctive and unlearned responses. Pavlov's work provided insights into how stimuli and environment can influence learned behaviors and reactions in humans and other animals.