Between AD326 and AD1582, Christianity determined Easter using an algorithm approved by a Church Council in AD325, with the equinox defined as March 21. From AD1054 (when the Orthodox and Catholic Churches split) through AD1582 both the Catholic and Orthodox Churches celebrated Easter on the same date, still using the algorithm from AD325. The Julian Calendar was used by the European (and Christan) communities until the Gregorian reform of 1582.
Since AD1582 October (when the Gregorian Calendar was adopted by much of Catholic Europe), the Orthodox Easter usually falls on dates different than the Western Christian Easter, although apparently the Churches are discussing using the same formula to determine Easter - probably a formula different than that currently used by either Church.
The Orthodox Easter is determined in the Julian Calendar. It has been claimed that Orthodox Easter does not fall on the date of Passover (15 Nisan in the Hebrew Calendar), or before it; this is true recently, but using the modern formulae for determining the date of Passover (rules which go back to the fourth century A.D.), one finds that, in fact, Easter occurred on the first day of Passover several times before the year A.D. 1000. From 1900 until 2099 the Eastern Easter will fall one (45.5%), four (4.5%), or five (21.5%) weeks after the Western Easter - and on the same date in 57 (28.5%) of those years. (I've compiled some Tables showing the offsets between Orthodox and Western Easters from 1583 through 3000 that shows this information.)