The category of dendroidal sets is an extension of that of simplicial
sets, geared towards providing a combinatorial model for the homotopy
theory of operads. The simplices forming the building blocks of a
simplicial set are replaced by "dendrices" which have automorphism
groups, while the cartesian product of simplicial sets is replaced by a
tensor product having some mysterious properties. The relation between
the homotopy theory of dendroidal sets and that of operads can be
expressed in a very precise way in terms of the theory of Quillen model
categories.