Kaddish, an Aramaic poem that praises God, is one of the oldest parts of the synagogue liturgy. It is also one of the most powerful and most enduring. Dating back to the first century, it was probably recited in the very first synagogues established after the destruction of the Second Temple in Jerusalem in the year 70 c.e. The central lines of kaddish are mentioned in the Talmud, which was written and edited in the third to the sixth centuries. The early rabbinic sources show kaddish associated with the study of sacred texts--it was said at the conclusion of Torah study-but by the Middle Ages, it became linked with mourning. At a certain point in the synagogue service, the head of the congregation would go outside where the mourners sat and say kaddish for them. Later, it was mourners who led the prayer. In his book "When a Jew Dies," Samuel C. Heilman writes that saying kaddish publicly "turns this prayer from a sterile mourner's monologue into a dialogue of praise of life." Rabbi Maurice Lamm, the author of "The Jewish Way in Death and Mourning," calls kaddish "a self-contained, miniature service that achieves the heights of holiness."
You can read a translation of the Mourner's Kaddish, with commentary, by following the link.