Daneel is a robot constructed to be indistinguishable from a human being (see android). This enables him to help earth-policeman Elijah Baley to solve crimes. (Daneel and Baley first meet while Baley is investigating the murder of Roj Nemennuh Sarton, the human from whom Daneel is indistinguishable.)
In his last books Asimov tied the Foundation Series and Robot series together to form a grand future history. In Foundation and Earth, the last of the series, we find out thet Daneel has survived for some 20,000 years and has manipulated the galaxy with the help of his many robot allies. He admits that he set up both the Galactic Empire and Gaia in order to create a society that did not need robots. Under the guise of Eto Demerzel, he also guided Hari Seldon so that he would develop psychohistory. Under the guise of both Chetter Hummin and Eto Demerzel, he is a very active character in Prelude to Foundation.