CPIO is the extension given to files used in UNIX-based operating systems. CPIO are files that are file archives. Technically speaking, CPIO is a type of container that can contain many files. The data in such a container is not compressed. Sometimes they are compressed with Gzip, but then the resulting files get the CPGZ extension.

CPIO archives contain, apart from the actual files, a header file that allows the archive to be restored correctly, eg using the GNU CPIO utility.