I answered Orthodox many ways just to have confusing looks - people would think Jew Orthodox and that doesn't blend properly with Easter, does it?
We are Eastern Orthodox (Greek Orthodox, Russian, Bulgarian, Ukrainian, Serbian, Romanian,and some African and Middle Eastern countries where they celebrate Orthodoxy too). We are Christians too, like Catholics and all its flavours. However the 2 churches separated in medieval times, in 1054 after the 'Great Schism' and separated they remained to these days.
There are a few notable differences in the theology of the 2 religions:
- the first is the absolute authority that the Pope has over the bishops: Eastern Orthodox did not agree with that (and that is the root of the Great Schism, which started with the excommunication of some Eastern bishops)
- the second - and most important theologically - is the 'Filioque': in the Eastern Creed we believe the Holly Spirit proceeds from the Father *and* the Son, while Catholic Creed has the Holy Spirit proceeding from the Father only. It is physically transferred in how we make the sign of cross: we put together 3 fingers (middle, index, and thumb), while Catholics only 2 (middle, index)
- the third is how each Church views the Virgin Mary: Catholics see her as born without sin, while Eastern Orthodox believe she was capable of sin, but resisted it successfully all her life. The view of sin is also different for ordinary people: Catholics believe in the original sin (Adam's) and that we are all guilty of it and we carry that guilt. Eastern Orthodox believe that we have inherited the sin, but we are not guilty of it. Catholics believe "all men have sinned through Adam's sin", while the Orthodox interpretation is "all humanity sins as part of the inheritance of the original sin". A subtle difference which says that we believe in the Original Sin and that all men have the propensity to sin, but they have not implicitly sinned through Adam's Original Sin. [confusing, eh?!]
- another interesting difference is on celibate: all Catholic Priests must maintain celibacy, while Eastern Orthodox must do so only if they have been ordained priests before marriage. Basically, a priest can marry if he does so *before* being ordained priests. The heads of the churches (bishops, archbishops etc.) are elected from the monks, though. But an ordinary priest can marry and have a family, which is quite nice and makes them very human and approachable.
For more details on the subject please check http://en.wikipedia.org/wiki/Eastern_Orthodox_–_Roman_Catholic_theological_differences.
The one everybody sees the most is the temporal difference: the time we celebrate holidays.
This is what happens: most of the Eastern Churches celebrate the 'moving' date holidays on the old calendar, and the fixed dates holidays on the new calendar. You might be aware originally everyone followed the Julian calendar. Then in 1582 Pope Gregor changed it to align the Easter to the date originally set at the First Council of Nicea - thus moving the calendar forward 10 days. A few other adjustments were made over the years - so today the civil calendar is moved 13 days in advance.
Funny fact: that's why what some of you might have heard of as 'The October Revolution' (the Russian one) is celebrated in November :)
However - the Easter countries were really slow in adopting the change (mainly because they were Orthodox and quite not following the rules of Catholics) - but they eventually did as countries and governments... not as Churches.
That's why we all think of March 11 as March 11 these days - but... when it comes to religious holidays we follow 2 different rules (yes, it's quite confusing, I know):
- the countries associated with Greek Orthodox keep the fixed date holidays (like Christmas) on the civil calendar - thus we celebrate on December 25th. However we keep the moving date holidays (like Easter) on the old Julian calendar - thus we celebrate God knows when, 'cause it's quite complicated to calculate :)
- the countries associate with Russian Orthodox keep *all* Holidays on the old, Julian Calendar. Thus they celebrate Easter when we do, but Christmas on January 6th.
And that is in a nutshell :)