The short answer: Easter falls on the first Sunday after the first full moon following the Spring Equinox.
|The long answer is REALLY long and literally covers thousands of years of history, arcane astronomical calculations and a little political wrangling.
For a well-documented discussion of the matter see: http://www.assa.org.au/edm.html.
This site details everything you ever wanted to know about Easter but were afraid to ask.
One interesting side note about the date of Easter is that the Eastern Orthodox churches do not celebrate Easter (called Pascha) on the same date as most other Christian churches. The Eastern Orthodox Church uses a different method to calculate the date and will generally fall a couple of weeks later than the western Easter.