Algebraic stacks are the geometric objects on which we will consider our D-modules and quasi-coherent
sheaves. For that reason it's necessary to know what they are. Below are some suggestions for the
reading:
1) The original paper by Deligne and Mumford, Sect. 4
2) "Champs algebriques" by Laumon and Moret-Bailly
3) Chapter 1 by A. Vistoli in the book
4) Notes by Dennis Gaitsgory and Toly Preygel