/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.
Clearly there are many more directories on the system—far too many to describe every one.
For changing things, you’ll usually want to confine yourself to your home directory and /etc. On a Debian system, there’s rarely an occasion to change anything else, because everything else is automatically installed for you.
/etc is used to configure the _system_ as a whole. You’ll use your /own home directory, a subdirectory of /home, for configuring your own preferences and storing your personal data. The idea is that on a day-to-day basis, you confine yourself to /home/_yourname_, so there’s no way you can break anything. Occasionally you log in as root to change something in a system-wide directory, but only when it’s absolutely necessary. Of course, if you’re using Debian at a school or business and someone else is the system administrator, you won’t have root access and will be able to change only your home directory and any other directory that you own. This limits what you can do with the system.