Saruman was at one time, a good guy. 
However, with his palantir (only seven
were made, and they are very mysterious),
he sees nothing but doom.  He decides that
the only way to stay alive in this war is
to join with Sauron.  He tries to convince
Gandalf, but he does not agree, and so he
traps the wizard in his tower (Orthanc)
He then creates an army -- a new breed of
orc (as if they couldn't get any worse)
called the Urukai.  However, as he was
treacherous to the Ents, they make an
attack on his tower, and he is trapped
inside.  He escapes however, and disappears
for the rest of the story...until....
the hobbits return to the shire as heroes
of war, even though the Hobbit population
doesn't know their great deeds.  Saruman
goes to the Shire and creates havok upon
the hobbits.  He is eventually overrun
though.  

I guess that's what you get for betraying
the good guys!