Adolescent rites of passage come in many shapes and forms, and are seen in cultures throughout the world and throughout history. Even what we think of as the traditional “Bar Mitzvah” is a relatively modern innovation — the even more recent innovation of “Bat Mitzvahs” (dating back to the 1920s) even moreso.
By definition, becoming bar/bat mitzvah is simply the act of turning thirteen. Originally, that simply meant this was the time in your life when you were ready to join the adult world, in life, in work, in worship.
In other words, the very concept of what a Bar/Bat Mitzvah (or, bar/bas mitsve, if you prefer the Yiddish transliteration) is something that, much like everything in our world, is constantly evolving.
The secular bar/bas mitsve was innovated by renowned academic/translator/poet Hershl Hartman. Its central conceit is that the (approximately) year of study preceding the joyous occasion is spent learning about and researching a topic of interest that is related in some way to the Jewish part of their identity. For students who come from a more multi-cultural background, this can also be an occasion to explore that part of their identity as well; for example, one particularly musically-gifted student with one Jewish parent and one parent from Appalachia, chose to do his bar mitsve project on the history of Klezmer and Appalachian music, including doing a klezmer performance on violin and a country and western number on piano.
Sample bar/bas mitsve presentations
The ceremony itself
Secular Jewish Clergy 