As far as Doctrine goes, absolutely nothing. The Church believes and teaches the very same thing. But the differences would be the fact that the Church and Her Doctrine has developed over time as the Church has continually strived to understand the mysteries of God. Those Doctrines, though believed ever since the Church began, but the details have over time become more understood as the Church has investigated them. That is development of Doctrine.
Very early on the Church had not developed a Bible canon, that is it had not determined what was inspired and what was not. But through Church Councils the Church did give a definition of what was canonical by the 4th century, and then at the Council of Trent the Church gave a final definition.
During the very early stages of the Church before Constantine, the Church faced severe persecution, and Christians were being slaughtered all the time everywhere. But Constantine lifted the persecution and the Church experienced a great deal of wonderful prosperity. The world has never been the same since, and the Church has played a big part in world affairs.
Church Councils are something that have similarity and differences. There has always been local Councils, but the Ecumenical Councils are a world wide council of the Church, and the first one was in 325 of Nicaea.