Wikipedia
Cramfs
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 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 first decompressing it. For this reason, some Linux distributions 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.
cramfs is obsolete, squashfs is the replacement.