FIFOFS (first-in first-out) 

Named pipe files that give processes common access to data

FDFS (file descriptors) 

Provides explicit names for opening files using file descriptors

NAMEFS 

Used mostly by STREAMS for dynamic mounts of file descriptors on top of files

SPECFS (special) 

Provides access to character special and block devices

SWAPFS 

File system used by the kernel for swapping