$S^6$ and the geometry of nearly K\"ahler $6$-manifolds

We review results on and around the almost complex structure on $S^6$, both from a classical and a modern point of view. These notes have been prepared for the Workshop “(Non)-existence of complex structures on $S^6$” (\emph{Erste Marburger Arbeitsgemeinschaft Mathematik – MAM-1}), held in Marburg in March 2017.