Spirulina - what is it?

Simply put, it is High Protein Food with lots of nutritional benefits.

It is a very high protein algae / planktonic cyanobacteria.  U.S Federal Drug Administration Talk paper # 41. pg 160. June 23rd 1981 states "Spirulina is a source of protein and contains various vitamins and minerals.  It may be legally marketed as a food or food supplement so long as it is  labelled accurately and contains no contaminated or adulterated substances."

At the risk of getting a bit confusing what is sold as Spirulina is technically not in terms of taxonmy.
The two cultivated varities are actually Arthrospira Platensis & Arthrospira Maxima and whilst the genus Arthospira shares the helical shape of the trichomes and the distribution of some species in the same selective habitats as Spirulina, Arthrospira is different and phylogenetically distant from Spirulina.   However the generic name Arthrospira is accepted as been valid for both A. maxima & A. platensis and also continues to be referred to by its commercial names Spirulina platensis & S. maxima.