Easter, also known as Resurrection Sunday, celebrates the resurrection of Jesus Christ. Christians believe that Jesus was crucified on Good Friday, and rose from the dead after three days on Easter Sunday. Easter Sunday marks the end of Lent, a 40-day period of fasting, prayer and penance, and is followed by a 50-day period called Eastertide, which ends with Pentecost Sunday.
The celebration of the resurrection of Jesus is observed as an Easter Vigil (also known as Paschal Vigil) in Roman Catholic, Eastern Orthodox, Lutheran and some Anglican churches, and "Sunrise Service" in many Protestant churches. Borrowing from the Jewish tradition, a liturgical day begins at sunset, and thus the Easter Vigil begins between sunset on Holy Saturday and sunrise on Easter Sunday.
The Easter Vigil service moves from darkness to light, symbolically re-enacting the Easter story of Jesus rising from the dead. The service begins outside the church where the priest lights and blesses a fire. A Paschal candle, representing the Risen Christ, is lit from that fire. The candle is processed through the church, and the Exsultet, Easter proclamation, is sung.
Passages from the Bible are read, the Gloria and Alleluia are sung, and the Eucharist (or Holy Communion) is celebrated. In many churches, this is also a time when new members are baptized into the Church in accordance with ancient tradition. The "Sunrise Service" is similar, but held outside early in the morning on Easter Sunday, so that attendants can see the sun rise.
Easter Sunday is preceded by Holy Week, which is the last week of Lent. Holy Week marks important events in Jesus's life as described in the Gospels -- his triumphal entrance to Jerusalem on Palm Sunday, the Last Supper on Maundy Thursday, and his death and crucifixion on Good Friday.
Like many big holidays, the celebration of Easter extends beyond the Church. Egg decorating, egg hunting and the Easter Bunny are examples of Easter customs that have extended into the secular world.