Anyone who is interested in learning about the tools (also known as commands) available to them when using their GNU/Linux based operating system.
Why would you want to learn how to use the command-line (and available tools)? The Command Line-Interface (CLI), while difficult to learn, is the quickest and most efficient way to use a computer for many different tasks. The CLI is the normal method of use for most UNIX system administrators, programmers and some power users. While a GUI is better suited to some tasks, many operations are best suited to the CLI.
The major motivation behind learning the GNU/Linux CLI is the authors idea that, with software in general, the more time spent learning something equals less time spent performing that particular task (authors opinion only).
This guide is aimed at beginners to intermediate users who want to learn about the command-line tools available to them. Advanced users may wish to use it as a command reference, however this document aims to list commands of interest, as judged by the authors opinion, it is not designed to be completely comprehensive, see the appendix, Section A.2.1 for further information. Or if you are not looking for a command reference guide, but a more gentle introduction to GNU/Linux you may be interested in the Introduction to Linux guide authored by Machtelt Garrels.
This guide could also be considered a summarised version of the Linux Cookbook. If you are looking for a book with more detailed descriptions of each tool have a look at the Linux Cookbook Homepage, also check out the command list from "Linux in a Nutshell 3rd Edition" for an index of 300+ commands and their explanations.