Saruman the white, like Gandalf the grey and Radagast the brown were part of the Istari. The Istari were Maia spirits (servents of the gods), sent from the west clad as old men to aid the free peoples of middle earth. There was very specific rules as to who and how they could help. Saruman was the leader of this council and he was called the white as was the colour of his hair. This is how they used age to symbolise wisdom in a way men could understand.
Sauron is also a Maia spirit though in the anceint days when the rebel God Melkor (aka Morgoth) began to pervert all things good in a desperate lust for glory, he and many other Maia turned to Melkor and supported his cause, for they too, I suppose lusted for the glory that had been reserved for only those they had once served. Of all the Maia Sauron was the most decietful and powerful, for Morgoth taught him many things and gave him many powers. When Melkor alas fell to the hands of brothers and sisters, Sauron hid in the depths of the earth and waiting seeking to avenge his master to finish the job Melkor has started.
Sauron then, after years of walking among the elves and men, clad in candy-coated lie and earning there trust devised a way to conquer them... a way to play there greed and trust against them... a way to bind them all with a single band of gold... and thus our story begins...
So they are both Maia, and slowly both became corrupted by power, but alas, Saruman the weakest, ironically refused evil the longest.