Random Walk: a basic model of diffusion (section 1.2.2) [a basic formulation of kinetic theory to derive formulas for the mean free path and the typical collision time is given in 1.2.1]
Brownian Motion: importance of separation between dissipative macroscopic time scale and collision microscopic time scale (first part of section 1.4)
Reference book: "Non equilibrium statistical physics: a modern
perspective" by Livi and Politi