An archive file format (and the program that reads it) commonly used on Unix and Linux.
tar (usual extension:
.tar) is an archive file format commonly used on Unix and Linux.
tar archives are informally called "tarballs".
tar is also the name of the command-line program used to read and write tarballs.
tar format itself doesn't support compression or encryption, tarballs are often compressed or encrypted using external programs. The resulting files conventionally have extensions like