/home This is where all normal users—that is, all users except root—have their home directories. Each home directory is named after the user who owns it, for example, /home/jane. If you’re using a large system at a school or business, your system administrator may create additional directories to contain home directories: /home1 and /home2 for example. On some other systems, you’ll see an additional level of subdirectories: /home/students/username, /home/staff/username, etc.
Your home directory is where you put all your personal work, e-mail and other documents, and personal configuration preferences. It’s your home on the system.
/bin This directory contains “binaries,” executable files that are essential to the operation of the system. Examples are the shell (bash) and file commands such as cp.
/sbin This directory contains “system binaries,” utilities that the root user or system administrator might want to use, but that you probably won’t want to use in your day-to-day activities.
/usr /usr contains most of the files you’ll be interested in. It has many subdirectories. /usr/bin and /usr/sbin are pretty much like /bin and /sbin, except that the directories in /usr are not considered “essential to the operation of the system.”
While not essential to getting the computer working, /usr does contain the applications you’ll use to get real work done. Also in /usr, you’ll find the /usr/man, /usr/info, and /usr/doc directories. These contain manual pages, info pages, and other documentation, respectively. And don’t forget /usr/games!
/usr/local The Debian system doesn’t install anything in this directory. You should use it if you want to install software that you compile yourself or any software not contained in a Debian package. You can also install software in your home directory if you’ll be the only one using it.
/etc /etc contains all the system-wide configuration files. Whenever you want to change something that affects all users of your computer—such as how you connect to the Internet or what kind of video card you have—you’ll probably have to log on as root and change a file in /etc.
/tmp Here you’ll find temporary files, most of them created by the system. This directory is generally erased on a regular basis or every time you reboot the system. You can create files here if you want, just be aware that they might get deleted automatically.
/var /var contains “variable” files that the system changes automatically. For example, incoming mail is stored here. The system keeps a log of its actions here. There are a number of other automatically generated files here as well. You’ll mostly be interested in the contents of /var/log, where you can find error messages that can help you figure out what you’re system’s up to if something goes wrong.