[The Sixth Wizard]

In terms of the story, you'd have to say that Sam and Frodo represent two different aspects of the "good" world which they fight to protect.

Sam is exemplary of the innocence, innate courage and kind-heartedness of those simple people of the Shire - it is his destiny to return and live a long, happy life, with fourteen lovely children. His role in the story is to illustrate that those who are pure, no matter how small or how simple (Sam isn't really that intelligent) are capable of standing up against evil. Tolkien says things like "even the smallest can achieve great things". Sam is never capable of the great deeds of Aragorn, Gandalf and Frodo himself - he fails to protect his master in the face of Shelob and Gollum, and he is easily outwitted and outmatched, but of course he doesn't fall prey to the temptation of the ring. In the end it is Sam's strength and purity of heart which allows Frodo to literally "climb the mountain" - perhaps Tolkien is trying to say that even the greatest achievements are always built on the dedication, literally on the backs of the smallest of us. While Smeagol and Frodo were pryers, looking for the secrets of the world, I think Sam was the more resistant to the Ring's powers because, as he says when he is tempted, he knows, deep down, that he could never be the Lord of the Ring. He is too humble and too simple for that. The Ring would consume him, given enough time, but it would have to destroy him first.