The story of the Orthodox church is kind of neat, if Rrhain will forgive me for jumping in.
The Council of Nicea (I'm about 90% sure that's right council) set up four patriarchs, who were basically in charge of final decisions for the church (one or perhaps two have been added since). That was the fourth century. The fall of Rome led to a geographical and political separation between the patriarch in Rome and the rest of the patriarchs, who were all in the east.
In the 9th century, the world situation restored better communication between the eastern and western churches and the eastern patriarchs and the western patriarch/pope. By then, the Roman patriarch had changed the Creed of Nicea (apostles' creed) by adding the phrase "and the Son" (Latin: filioque) to the phrase "We believe in the Holy Spirit, who proceeds from the Father."
The patriarchs of the east argued for 200 years that the Roman patriarch couldn't do this without permission of the others. The Roman patriarch said he could. After 200 years they gave up arguing and split ("The Great Schism").
It was all over one latin word (three English words). A couple years ago, Pope John Paul II recited the Nicene Creed without the filioque, and it was taken note of.
The Orthodox churches and the Roman Catholic church have been "evolving" mostly separately since late in the 4th century. There are a lot of differences that wouldn't be significant to outsiders, but are extremely significant to informed members, especially if they're Orthodox.
The Orthodox aren't allowed to make big doctrinal changes, because, in their opinion, they don't have all the patriarchs. One is fallen away. The Romans can make all the doctrinal changes they want, because they only recognize one of the patriarchs.
The two groups have different rites, baptize differently, view the Trinity differently, Mary differently, and salvation differently. The Orthodox consider the use of statues to be idolatry. They use icons, which are two dimensional and not exact replicas (they're more cartoon like).
Real committed Orthodox believers can be very rude, a lot like fundamentalists. To the Orthodox, Protestantism is just a part of Catholicism, all of them equally separated from the true catholic (universal) Church.