A graph is said positively multiplicative when its adjacency matrix A embeds in a matrix algebra admitting a basis B with nonnegative structure constants in which the matrix multiplication by A coincides with A. The goal of this paper is to present basics on this notion and expose, through various simple examples, how it relates to highly non trivial problems like the combinatorial description of fusion rules, the determination of spectrum in Cayley graphs, the description of the minimal boundary of graded graphs or the study of random walks on alcove tilings.