The compressed ROM file system (or cramfs) is a free (GPL'ed) read-only Linux file system designed for simplicity and space-efficiency. It is mainly used in embedded systems and small-footprint systems.
Unlike a compressed image of a conventional file system, a cramfs image can be used as it is i.e. without the need to decompress the image first. For this reason, some Linux distributions also use cramfs for initrd images (Debian 3.1 in particular) and installation images (SUSE Linux in particular), where there are constraints on memory and image size.