One name, many books.
The history of Torah is one of interpretation. Every seemingly superfluous letter, unclear transition, and difficult phrase invites discussion, explanation, and elaboration. This interpretive tradition has produced volumes and volumes of midrash--stories, homilies, parables, and legal exegesis based on the biblical text. These texts offer a glimpse of the ways that people of various times and places have grappled to understand the biblical text and to make it meaningful for their own lives.
The body of literature known as midrash is generally divided into aggadic (narrative) and halakhic (legal) midrash. Collections that contain mostly stories, parables, and homilies are classified as midrash aggadah, while collections focused primarily on the derivation of law are called midrash halakhah.
The largest volumes of midrash aggadah are often referred to collectively as Midrash Rabbah. This name is actually a misnomer, as this group of texts comprises ten unrelated collections, compiled over the course of eight or more centuries. Each volume comments on one of the five books of the Torah (Genesis, Exodus, Leviticus, Numbers, and Deuteronomy) or of the five Megillot (Song of Songs, Ruth, Lamentations, Ecclesiastes, and Esther).
Some scholars trace the name Rabbah to the first line of B'reishit Rabbah (Genesis Rabbah), which begins, "Rabbi Oshaya Rabbah opened."
Just as books of the Bible often draw their names from the first significant word of the text, this book of midrash also seems to have acquired the name of the first rabbi quoted in it. Others argue that the title Rabbah, which means "great" or "large," is intended to distinguish this book from a smaller volume that must once have existed. Whatever its origin, the term "Rabbah" later came to be applied to the largest collections of aggadic midrash on each of the five books of the Torah and the five Megillot. In turn, shorter collections of aggadic midrashism on a few of these books acquired the designation, Zuta, which means "small" in Aramaic.
B'reishit Rabbah and Vayikra Rabbah
The oldest extant aggadic midrashimare B'reishit (Genesis) Rabbah and Vayikra (Leviticus) Rabbah. Both were probably compiled around the fifth century CE, but each includes material dating back at least to the third or fourth century. These midrashim are written in a combination of Hebrew and Aramaic, and are peppered with Greek words and expressions.
B'reishit Rabbah consists of a mixture of line-by-line commentary, parables, popular sayings, and legal principles. Many of the best known midrashim, including the story of Abraham breaking his father's idols, appear in this collection.