"GNU tar is an archiver that creates and handles file archives in various formats. You can use tar to create file archives, to extract files from previously created archives, store additional files, or update or list files which were already stored." (from http://directory.fsf.org/tar.html).
The ipk version is the complete tar, and not the smaller Busybox version. For most people, the Busybox version should be perfectly fine. However, if you intend to build packages natively on the slug,
If you get error messages when creating or extracting a tar archive ("tar: Names longer than 100 chars not supported."), you are still using the Busybox version of tar. Log in as root and do