While he does get possessed by a demon (the yellow-eyed demon, Azazel) in the very first episode and later by Lucifer, he himself is not a demon.
Sam is a human with special abilities that come from his lineage (he's the son of John Winchester and Mary Winchester, who were hunters). His special abilities include a heightened sense of smell, strength, and resilience.
It's important to note that his character is complex, and he does face temptations and struggles throughout the series that could make him question his own humanity. However, he remains a human hunter, fighting against evil forces, including demons.