I went through a time of reading about these guys. And I knew a man who was a member, and another who almost joined, but didn't.
Soka Gakkai used to be the most prominent form of Nichiren Buddhism in America, and it may still be, but it's not the only form of Nichiren Buddhism there is. Nichiren, like most religions of any age at all, has divisions and subdivisions. You can watch the back and forth from different parties on the Internet, if you like.
As the Wiki article Kissa links to says, the focus of NB is on the Lotus Sutra, and the daily chanting is mainly of a mantra that pretty much means, "Hail Lotus Sutra!" (This mantra actually figured in an episode of the old TV show, The Monkees.) There is sometimes also chanting from the sutra itself. The main object of veneration is a copy of a calligraphic mandala with that mantra in the center, and the names of various dieties surrounding it. Nichiren, the Japanese monk who started this school of Buddhism, made several of these.
NB is the form of Buddhism that has attracted the most African Americans, out of all the forms of Buddhism practiced here. Another curious thing about this group is that its original base in the US was among military personnel who had been stationed in Japan and converted there. A couple of prominent American Nichiren Buddhists are Tina Turner and Patrick Duffy (Bobby Ewing from Dallas).
It's been a while since I was reading about this so I'm a bit rusty at it. As I recall, Japan is very important in the NB view of things. Which can give it a nationalistic cast, depending on who is presenting it. On the other hand, there's an entire world of Buddhism corporately referred to as "Tibetan Buddhism," so that's not really so remarkable.