The contents of this book can be divided into two topics: the core C language, and the standard functionality made available to the programmer. The standard functionality I mention is provided by GNU Libc, this is a library of C functionality that is part of every GNU system. Neither of these topics is of much use without the other but there is a focus on the core langauge near the beginning and more discussion on Libc near the end. The ordering of topics is designed to teach C programming in an incremental fashion where each chapter builds on the previous one. Some aspects of the core langauge are only really of use to experienced programmers and so appear near the end.
The C language on it's own can make decisions, repeat commands, store data, and perform mathematics. Equally importantly, it provides a method to make use of extra functionality such as Libc.
Libc provides functionality such as reading and writing files, sorting and searching data, getting input from the user, displaying data to the user, communicating across networks, creating easily translatable programs, and many other things.