A.. (ahem) mentally handi-capable (that being the 'politically correct' term,) individual, whom, even thouhg they may be unable to say the most basic of sentences, are geniuses at certain areas. And idiot savant can play the piano like a pr0, but may not be able to say his/her own name. Other such areas include painting, ect.
Peter Griffin is an idiot savant.
Though he is mentally handi-capable, he can play the piano beautifully (albeit only when he is drunk).