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.