What is the preface to Lanting Collection also called in calligraphy, and what genre does it belong to?

Lanting Preface is the first running script in the world, which was written by Wang Xizhi. At present, there is no original work. It is supposed that it has become a funerary object. Now Lanting Preface is described by later generations, and there are many versions. As for the genre, is it that Wang Xizhi played with friends and wrote a preface in Lanting, the content of which is the experience of playing and his own feelings, which belongs to the running script?