Here are the top level directories in your Linux system, with a brief explanation of the purpose of each:

/ : The top level directory in your system. It's called the root directory, because it's the root of the system: all the rest of the directory structure emanates from it like branches from the root of a tree. Don't get confused by the /root directory, by the way; that's a different 'root'.

/bin : Contains binaries for your system. The /bin directory is, by default, in your PATH, meaning that any executable file in this directory can be executed just by entering the file name at the command line.

/boot : Contains files necessary to boot your system, including the kernel.

/dev : An abstracted directory which contains all your system devices: hard drives, CD-ROMs, sound cards, and much, much more. When devices are mounted, it occurs here.

/etc : Contains most of the system and application configuration files your Linux machine uses. Back this directory up regularly.

/etc/skel : "skeleton" files used to create user accounts.

/home : Contains the home directory for each system user.

/lib : System modules, software libraries, & information databases, shared by various applications & the system itself.

/opt : Optional software that you installed.

/proc : Another abstracted directory which is created when the system boots. Contains information about the processes on your system.

/root : The home directory of the root user.

/sbin : Contains additional system binaries that are only available to root. Mostly admin tools.

/tmp : The temporary directory. Any user can place stuff in here, but be forewarned: your system periodically cleans out this directory, and anything you leave in here will be deleted.

/usr : Contains subdirectories that will be used system-wide by all users, including binaries, libraries, man files, and other things.

/usr/doc : Program documentation.

/usr/info : Program documentation.

/usr/lib : Program libraries (shareable system program modules).

/usr/man : Manual pages.

/var : Contains variable data that changes in size as the system runs. For instance, log files, mail directories, databases, and printing spools.