In Lord of the Rings, Saruman The White, the eldest of the Wizards and their chieftain, joins with Sauron instead of helping people fight against Sauron. What is the reason for him losing faith in Gandalf, the Elves and the men and becoming an ally of Sauron?
This is fully explained in the Istari material in Unfinished Tales, with the most direct information being that he:
This is also expressed in Letter 181:
So in other words, Saruman started out with good intentions but his pride and eagerness to get a quick result got the better of him.
Lord of the Rings itself makes clear that one of his motives was desire for possession of the Ring, and - when we consider the above quote - it seems evident that this desire was initially as a means of power for the over throw of Sauron, but following his lapse became a desire for his own personal power.
Saruman himself explains it all in Gandalf's story at the Council of Elrond:
This also introduces a new dimension that's important in Tolkien's works: the abandonment of hope and giving into despair. Saruman saw no way of winning through direct confrontation and so succumbed to an alternative path that would eventually destroy him.
So that's a combination of the following main factors:
Saruman, by the way, comes with a flawed pedigree. He was in origin a Maia of Aule, as was Sauron (they would have probably known each other well before Sauron was corrupted in the Elder Days), and Aule himself had (semi) rebelled against Ilúvatar's will in his making of the Dwarves. There's room for interesting speculation there, but perhaps not appropriate for inclusion in an answer.
Saruman was sent to Middle Earth in order to help the Free peoples defeat Sauron.
In order to fight against Sauron he had to study the powers of the One Ring. When he did, he saw how powerful the one ring was and wanted it for himself. He had begun to sense the resurgence of Sauron and to envy and desire his power, and especially his One Ring.
-- J.R.R. Tolkien, The Lord of the Rings, Appendix B, "The Third Age"
Simple Answer: He wanted the ring for himself, once he saw how powerful it actually was