A: Tar is a program to combine files in a directory system into a single file. The result is called a TAR file. Another common name for this TAR file is "tarball". NOTE: if you are familiar with Java language's "jar" facility, then you can view "jar" as Java-specific derivative of "tar".
A: A TAR file is very useful for archiving purposes, and for sending the entire directory of files via email or other transport programs (e.g., ftp).
In my classes, I often require students to submit programming homework in a TAR file; this will typically contain the program files, data files, a Makefile and a README file.
A: Yes, there many, especially on Windows systems. HOWEVER, they are generally incompatible with tar, and a hassle to deal with in a non-windows environment. Hence, please do not send such files if I ask for TAR files.
A: The implicit understanding of this question is that the tar program is mainly used in UNIX like systems. This is true. But it is also freely available on Windows sytems. There are many ways to get such:
BUT please do not use "tar-like" software where is not compatible with "tar". E.g., "rar" is common in Windows. There are many such variants.
Please send comments to yap (at) cs (dot) nyu (dot) edu.