I got this from www.care2.com:
The Story of Passover
Passover, (or Pesach, which means "Passing Over" or "Protection"), originated as a spring nature festival in ancient times, and gradually changed into a memorial of the Jews' springtime deliverance from the land of Egypt, where they were slaves (under the rule of the Pharoah Ramses II, according to tradition).
According to the story, the patriarch Moses pleaded with Pharoah to release the People of Israel from their bondage. His demand was refused repeatedly until God unleashed a series of fearsome plagues upon the Egyptians, including rains of blood and frogs, hailstorms, lice, disease, various forms of vermin and the untimely death of first-born Egyptian children. The Israelites protected themselves from these plagues by marking their dwellings with the blood of a lamb.
Relenting under such pressure, Pharoah agreed to allow them to leave Egypt. The Jews moved quickly, not even taking the time to bake bread. Instead, they brought raw dough to bake into unleavened bread as they fled.
Their haste was well considered as Pharoah quickly went back on his word and pursued the retreating Israelites into the desert, where they were trapped between the Egyptian army and the Red Sea. God parted the waves and exposed a stretch of dry riverbed for the Hebrews to cross. When the Egyptians pursued them, the river resumed its normal course, drowning Pharoah's army as the waters closed over them.
Passover celebrates this history with an eight-day observance during which special dietary rules are followed. Only food that is "Kosher for Passover" is allowed. Any foods prohibited during the Holiday must be disposed of by the morning of the first night. No leavened (made with yeast) foods or grains are eaten. During this time, Jews eat unleavened bread to remember their hurried escape from Egypt. The kitchen utensils and dishware used for daily food preparation are not be used during Passover. Rather, special dishes and utensils specifically intended for the holiday are taken out of storage, cleaned and used in their place.
Seders, or special dinners, are held on the first two nights to commemorate the mass exodus. The seder includes bitter herbs (horseradish) to commemorate the harsh life of slavery, and other foods of ritual significance. Seders can last into the night as families read and discuss the Haggadah, or Passover liturgy. Passover lasts for eight days, with observant Jews not working or sustaining usual activities during the first two and the last two days.