A filesystem is a subsystem of a UNIX system. It is built on top of secondary storage devices but not limited to that and can also be built on top of primary storage devices (RAM) or be an entirely virtual filesystem. A UNIX system cannot operate without bootstrapping itself and programs from the filesystem.
All filesystems available in a UNIX system are mounted in a tree hierarchy that starts with the root filesystem at the top and branches out to other filesystems toward the branches. The structure of the entire filesystems tree is made up of directories with files, character devices, block devices, sockets and symlinks making up the branches along the tree. From the root to the farthest branch from the root only 1023 bytes may be spanned in writing the path delimited by '/' characters, between the '/' characters a directory or file may only be up to 255 bytes long.
/ /bin /sbin /etc /etc/X11 ... /usr /usr/bin /usr/sbin ... /usr/local /usr/local/bin /usr/local/sbin /mnt /tmp /dev
List of filesystems available in UNIX and UNIX-like Operating Systems
Unix File System (ufs)
Berkeley Fast File System (FFS)
Memory File System (mfs)
Proc File System (procfs)
The procfs filesystem is a filesystem that represents the process namespace of a system. You can see all processes listed here and when you change into the directories of the processes and list further attributes about the specific process.