Arianism is the nontrinitarian, theological teaching attributed to Arius (c. AD 250–336), a Christian presbyter in Alexandria, Egypt, concerning the relationship of God the Father to the Son of God, Jesus Christ. Arius asserted that the Son of God was a subordinate entity to God the Father. Deemed a heretic by the Ecumenical First Council of Nicaea of 325, Arius was later exonerated in 335 at the regional First Synod of Tyre, and then, after his death, pronounced a heretic again at the Ecumenical First Council of Constantinople of 381. The Roman Emperors Constantius II (337–361) and Valens (364–378) were Arians or Semi-Arians.
The Arian concept of Christ is that the Son of God did not always exist, but was created by—and is therefore distinct from—God the Father. This belief is grounded in the Gospel of John (14:28) passage: "You heard me say, 'I am going away and I am coming back to you.' If you loved me, you would be glad that I am going to the Father, for the Father is greater than I." .....
By 325, the controversy had become significant enough that the Emperor Constantine called an assembly of bishops, the First Council of Nicaea, which condemned Arius' doctrine and formulated the original Nicene Creed of 325. The Nicene Creed's central term, used to describe the relationship between the Father and the Son, is Homoousios (Ancient Greek: ὁμοούσιος), or Consubstantiality, meaning "of the same substance" or "of one being". (The Athanasian Creed is less often used but is a more overtly anti-Arian statement on the Trinity.)
http://en.wikipedia.org/wiki/Arianism