This guide assumes you have access to the shell on a Linux computer. All the examples use a Raspberry Pi running Raspbian. If you haven't, you should start with What is this "Linux", anyhow? and What is the Command Line?
This is Adventure, a game almost as old as Unix itself, and much older than Linux:
Like the shell, Adventure's interface is made entirely of text.
You interact with the world of the game by typing commands and reading responses. ENTER BUILDING or GET LAMP, for example.
The shell is a lot like that game world, except that instead of a map full of rooms and objects, you navigate a computer's filesystem and work with the files it contains.
The filesystem, everything the operating system needs to boot up, run software, save files, take photos, etc. is organized into directories. We usually call these "folders" in the Windows and Macintosh worlds, but the concept is the same. A directory is just a special kind of file that can contain other files, including directories.
As a user, you have a home directory. In the shell, ~ (the tilde) is a common shorthand for this. In this case, it points at /home/pi.
Log in to your Pi, acquire a terminal, and verify this:
pwd stands for print working directory. Commands in the Unix tradition tend to have short, cryptic names. Some can be easily remembered because they stand for something obvious. Others just have to be committed to memory.
If you have a look around your Pi, you'll find that it contains all sorts of stuff. You just need to know a few more commands. The following pages provide quick introductions to some of the basics.