Easter is always the 1st Sunday after the 1st full moon after the Spring Equinox (which is March 20). This dating of Easter is based on the lunar calendar that Hebrew people used to identify Passover, which is why it moves around on our Roman calendar.
Computus is the term used for the calculation of the date of Easter. Easter is the first Sunday after the 14th day of the lunar month (that is the day of the Full Moon) that falls on or after 21 March.
This method was agreed upon at the council of Nicene (325 A.D.) Because not all lunar months have the same number of days, and because the calculation is tied to the vernal equinox, not to mention the variances of leap years in the solar calendar the actual computation of the Day of Easter is moderately complex. The Eastern Churches, which have a few additional rules, and use the Julian, not the Gregorian calendar celebrates Easter on a different day than the west.
A decade ago there was a move among the Protestant Churches to change the way the date of Easter was calculated. Nothing came of it and all western Christian churches continue to follow the method proscribed by the Catholic Church almost 1700 years ago.
Now for the interesting bits: