In computing, tar (derived from tape archive) is both file format (in the form of a type of archive bitstream) and the name of the program used to handle such files. The format was standardized by POSIX.1-1988 and later POSIX.1-2001. Initially developed as a raw format, used for tape backup and other sequential access devices for backup purposes, it is now commonly used to collate collections of files into one larger file, for distribution or archiving, while preserving file system information such as user and group permissions, dates, and directory structures.
tar's linear roots can still be seen in its ability to work on any data stream and its slow partial extraction performance, as it has to read through the whole archive to extract only the final file. A tar file (somefile.tar), when subsequently compressed using a compression utility such as gzip, bzip, or (formerly) compress, produces a compressed tar file with a filename extension indicating the type of compression (e.g.: somefile.tar.gz). A compressed .tar file is commonly referred to as a tarball.
As is common with Unix utilities, tar is a single specialist program. It follows the Unix philosophy in that it can "do only one thing" (archive), "but do it well". tar is most commonly used in tandem with an external compression utility, since it has no built-in data compression facilities. These compression utilities generally only compress a single file, hence the pairing with tar, which can produce a single file from many files. To ease this common usage, the BSD and GNU versions of tar support the command line options -z (gzip), and -j (bzip2) which will compress or decompress the archive file it is currently working with, although even in this case the (de)compression is still actually performed by an external program. Compression is sometimes avoided because of the greatly amplified potential for damage to data in long term storage.

Format details
The file header block contains metadata about a file. To ensure portability across different architectures with different byte orderings, the information in the header block is encoded in ASCII. Thus if all the files in an archive are text files, then the archive is essentially an ASCII file.
The fields defined by the original Unix tar format are listed in the table below. When a field is unused it is zero filled. The header is padded with zero bytes to make it up to a 512 byte block.
The Link indicator field can have the following values:
A directory is also indicated by having a trailing slash(/) in the name.
For historical reasons numerical values are encoded in as ASCII text octal numbers, with leading zeroes. The final character is either a null or a space. Thus although there are 12 bytes reserved for storing the file size, only 11 octal digits can be stored. This gives a maximum file size of 8 gigabytes on archived files. To overcome this limitation some versions of tar, including the GNU implementation, support an extension in which the file size is encoded in binary. Additionally, versions of GNU tar from 1999 and before pad the values with space characters instead of zero characters.
The checksum is calculated by taking the sum of the byte values of the header block with the eight checksum bytes taken to be ascii spaces (value 32). It is stored as a six digit octal number with leading zeroes followed by a nul and then a space.

File header
Most modern tar programs read and write archives in the new USTAR (Uniform Standard Tape Archive) format, which has an extended header definition as defined by the POSIX (IEEE P1003.1) standards group. Older tar programs will ignore the extra information, while newer programs will test for the presence of the "ustar" string to determine if the new format is in use. The USTAR format allows for longer file names and stores extra information about each file.

Example
Tarbomb is derogatory hacker slang used to refer to a tarball containing files that untar to the current directory instead of untarring into a directory of their own. This can be a potential problem if it overwrites files using the same name in the current directory. It can also be a pain for the user who then needs to delete all the files that are scattered over the directory amongst other files. Often this ends up happening in the user's home directory. Such behaviour is often considered bad etiquette on the part of the archive's creator.

Tarbombs
Tarpit is a term to describe a method of revision control where a tar is used to capture the state of development of a software module at a particular point in time. The use of a tarpit typically loosely mirrors the use of a revision control software tag and branching through the use of descriptive names.

Notes

List of archive formats
List of file archivers
Comparison of file archivers
List of Unix programs