This document discusses data structures and the C preprocessor. It defines data structures as organized ways to store data in a computer. Common data structures include arrays, stacks, queues, linked lists, and trees. The C preprocessor processes C programs before compilation by performing tasks like including header files, defining macros, and conditional compilation. Preprocessor directives start with # and are used to transform the code before the compiler sees it. Macros are a common preprocessor feature that allows text substitutions using identifiers.