The Haggadah is the text which is used for the Seder, and which has been used for at least a thousand years, though its elements are clearly mentioned in the Talmud, and the Seder itself goes back to the first Pesach, celebrated in Egypt.
Who was its author?
The actual author is unknown, but the "virtual" author is referred to as the "Baal HaHaggadah," the "Author of the Haggadah. It was probably put into its present form in the time of the Geonim, leaders who served the Jewish People for several hundred years in Babylonia, until about the year 1000. The oldest Haggadah was found in the Siddur of one of the last, and greatest, Geonim, Rabbi Saadiah Gaon.
How many editions have appeared?
Since the fifteenth century, more than 2700 editions have appeared, some with hundreds of commentaries.
What are the contents of the Haggadah?
It is a collection of excerpts from the Bible, the Mishnah and the Midrash, interspersed with numerous rituals, such as the eating of Matzah and the drinking of four cups of wine. (This is the only night of the year (two nights in the Diaspora, where two Sedarim are held) except for Purim, when more than one cup of wine is good for the body and the soul.)
Take a sip of a wine-connection between Purim and Pesach!
It is to a large extent in Question and Answer format. This is done to help parents and grandparents educate their children and grandchildren (or, possibly, the reverse) in the Story of the Exodus, and the Story of Jewish History and Destiny.
What is the Purpose of the Haggadah?
It is the core of the uniquely spell-binding and transcendent experience which the Seder is for Jews knowledgeable enough to appreciate it. And it has the power, beyond that of any best-selling book, with the exception of the Hebrew Bible and the Hebrew Prayer Book, the Siddur, to "bring back" Jews who, for one reason or another, have temporarily lost contact with their roots.
One of the most important statements in the Haggadah is the following:
"In every generation, an individual Jew is obligated to view himself as having left Egypt, as it is written, 'And you shall tell your child on that day that this is on account of what the L-rd did for ME when I came out of Egypt.' It was not only our fathers whom the Holy One Redeemed from slavery; we, too, were Redeemed with them, as it is written, 'He took US out from there so that He might take US to the Land which He had sworn to our fathers.' "