The hallmark of Nichiren Buddhism is the mantra Namu Myoho Renge Kyo. Nichiren Buddhists chant some version of this phrase, which is known as daimoku or Odaimoku.
"Myoho Renge Kyo" is the title of the Lotus Sutra, and "Namu" means devotion or reverence. Chanting "Namu Myoho Renge Kyo" is, therefore, to embrace the Lotus Sutra.
Aside from that, Nichiren Buddhists -- comprising dozens of schools, sects, so-called new religions and peace movements -- agree on little else.
Nichiren was a Buddhist monk who lived in thirteenth century Japan. His teachings have endured for more than 750 years, mostly in Japan. While some pioneers introduced Nichiren and his teachings to the west long before World War II, it was only after the war that proselytizing and conversion to Nichiren's teachings became wildly popular in Japan, and the practice was exported with messianic fervor to the United States.
A summary of Nichiren's central teachings can be found in a short letter he wrote known as On Attaining Buddhahood in this Lifetime. While some people believe that this letter was written by one of Nichiren's followers rather than by Nichiren himself, it will give you a sense of what Nichiren Buddhism is about.
In the U.S., Nichiren Buddhism has an undeserved reputation as a form of quack spirituality. The mantra has been touted as a magic spell or lucky formula. But it's not. Chanting daimoku is a centuries-old practice with an extensive body of literature to support it.
Nichiren Buddhists have a much-deserved reputation for fighting with one another -- between and within sects -- and for dissing other Buddhist teachings and traditions. There are reasons for this contention and discord, but they tend to make newcomers' eyes glaze. It's important to note that nothing about Nichiren Buddhism demands or excuses crappy behavior by its practitioners.
The main thing about Nichiren Buddhism is: Hang in there. If you feel an affinity for the mantra -- or even a passing interest in the practice -- take it and make it your own.
For specific suggestions about how to practice, check out Reverend Ryuei's practice instructions for Nichiren Shu.
Listen to MP3 Samples of Chanting:
Nichiren Shu Shodai (Namu Myoho Renge Kyo)
Nichiren Shoshu and Soka Gakkai Shodai (Nam-myoho-renge-kyo)
Nichiren Shu Prolonged Daimoku (known as "hiki daimoku" or "sansho")
Listen to Yoko Ono's song Nammyohorengekyo on YouTube.
Read the Lotus Sutra.
Introductory Explanation
Cris Roman's book-in-progress offers a solid introduction to Nichiren Buddhism. (Please note that the book reflects a Fuji School understanding -- other schools and sects understand Nichiren Buddhism differently.) It's a good place to start:
Chapter 1: Looking for a Bridge
Chapter 2: Nichiren and the Lotus Sutra
Chapter 3: Defining Nam-myoho-renge-kyo
Chapter 4: The Benefits of Buddhist Practice
Chapter 5: A Focal Point for One's Faith
Chapter 6: The Gohonzon and Bodhisattva Practice
Chapter 7: A Personal Relationship with the Gohonzon
Chapter 8: Theoretical Underpinnings of the Gohonzon
Chapter 9: Theoretical Underpinnings of the Gohonzon, Part Two
Chapter 10: Gongyo, An Intensely Personal Symphony
Chapter 11: The Moment of Death, and Changing Karma
Chapter 12: Total Responsibility