Every formatted hard drive partition has a file system on it. These file systems are not required to be the same. In Windows each file system is mounted to a different drive letter, thus starting a new directory tree, whereas on Linux one file system is mounted on root i.e "/" and other file systems are mounted on subdirectories of the root file system, creating a single directory tree. The file system mounted on "/" is called Root File System.
Given below are the most important subdirectories of root directory:
This directory contains the programs(binaries) that are required during bootup. These programs may be run by root user as well as a normal user. There must be no subdirectory in /bin.
This directory is similar to /bin as it also contains binaries that are required during bootup but these binaries can only be run by super user i.e root user.
This directory contains library files that are required by different binaries in /bin and /sbin.
Similar to /bin except that it contains programs which are not required during bootup.
Similar to /sbin except that it contains programs which are not required during bootup.
This directory is analogous to "Documents & Settings" directory in Windows. It contains 1 directory per user known as home directory of that user. All user specific files, configuration, documents and etc are stored in the users home directory.
This directory contains boot loader files. Boot loader is a program that boots Linux.
This directory contains system wide configuration files e.g network configuration, XWindows configuration and etc.
This directory contains variable files i.e the files whose content changes continually during the operation of a system. An example of such files are log files which can be found at /var/log directory
More on Linux Directory Structure can be found at Filesystem Hierarchy Stadard website and on Wikipedia