Doctrine

There's a reasonably good summary on the Council of Nicea at LiveScience. The writer shows small appreciation for the implications of Arianism's divergence from orthodoxy, but in such a short piece, there's hardly room for all that anyway. The bit about the Son being of the same substance doesn't really do justice to the earlier part of the Nicene Creed's second article: "...God of God, Light of Light, Very God of Very God; begotten, not made..."

It is worth noting that from a secular-historical point of view, Arians were Christians, and thus the Christian Church at the time was possibly more Arian than orthodox, if counted democratically. From a theological point of view, however, Christians are defined by doctrine, not by labels alone. This might be hard for some of our contemporaries to grasp, but it has been the Christian approach from the Beginning. Therefore, the Arians were not Christians, just as their present-day counterparts (Jehovah's Witnesses, Mormons, and the like) are not Christians.