http://mb-soft.com/believe/txo/arianism.htm
Excerpt from the site:
Arianism was a 4th-century Christian heresy named for Arius (c.250-c.336), a priest in Alexandria. Arius denied the full deity of the preexistent Son of God who became incarnate in Jesus Christ. He held that the Son, while divine and like God ("of like substance"), was created by God as the agent through whom he created the universe. Arius said of the Son, "there was a time when he was not." Arianism became so widespread in the Christian church and resulted in such disunity that the emperor Constantine convoked a church council at Nicaea in 325 (see Councils of Nicaea).
Led by Athanasius, bishop of Alexandria, the council condemned Arianism and stated that the Son was consubstantial (of one and the same substance or being) and coeternal with the Father, a belief formulated as homoousios ("of one substance") against the Arian position of homoiousios ("of like substance"). Nonetheless, the conflict continued, aided by the conflicting politics of the empire after the death of Constantine (337).
Three types of Arianism emerged: radical Arianism, which asserted that the Son was "dissimilar" to the Father; homoeanism, which held that the Son was similar to the Father; and semi-Arianism, which shaded off into orthodoxy and held that the Son was similar yet distinct from the Father.
After an initial victory of the homoean party in 357, the semi-Arians joined the ranks of orthodoxy, which finally triumphed except in Teutonic Christianity, where Arianism survived until after the conversion (496) of the Franks. Although much of the dispute about Arianism seems a battle over words (Edward Gibbon scornfully observed that Christianity was split over a single iota, the difference between homoousios and homoiousios), a fundamental issue involving the integrity of the Gospel was at stake: whether God was really in Christ reconciling the world to himself.