UNIX machine is connected to several file systems, each with its own i-list. One of those i-lists points to a special storage area, known as the root file system. The root file system contains the files for the operating system itself, and must be available at all times. Other file systems are removable. Removable file systems can be attached, or mounted, to the root file system. Typically, an empty directory is created on the root file system as a mount point, and a removable file system is attached there. When you issue a cd command to access the files and directories of a mounted removable file system, your file operations will be controlled through the i-list of the removable file system.

Each file system that is mounted on a UNIX machine is accessed through its own block special file. The information on each of the block special files is kept in a system database called the file system table, and is usually located in /etc/fstab. It includes information about the name of the device, the directory name under which it will be mounted, and the read and write privileges for the device. It is possible to mount a file system as "read-only," to prevent users from changing anything.

Example: Important File system Used by HP-UX

/hp-ux
The kernel program

/dev/
Where special files are kept

/bin/
Executable system utilities, like sh, cp, rm

/etc/
System configuration files and databases

/lib/
Operating system and programming libraries

/tmp/
System scratch files (all users can write here)

/lost+found/
Where the file system checker puts detached files

/usr/bin/
Additional user commands

/usr/include/
Standard system header files

/usr/lib/
More programming and system call libraries

/usr/local/
Typically a place where local utilities go

/usr/man
The manual pages are kept here


Also pls do re-define your question on 'format' Is it a filesystem format? floppy format? hard disk format?