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.
To view files in a filesystem the command ls is used.
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.