Unrar is the name of two different command-line applications for extracting RARarchives.
RARLAB UnRAR
This version of UnRAR is distributed by RARLAB, the makers of the commercial WinRAR archivers. It can extract newer RAR3 archives not supported by GNA's version.
GNA UnRAR
This free software version of UnRAR is based on an old version of RARLAB's UnRAR with permission from author Eugene Roshal. It is licensed under the GPL. It does not support the RAR3 format.
The Unarchiver unar
The Unarchiver is a replacement for "Archive Utility.app" in Mac OS X and is free software. Its command line Version unar/lsar supports the RAR3 format and is part of many GNU/Linux Distributions. Since Version 3.6 the GNOME unarchiving utility File Roller supports unrar.