1. Generally speaking, "to mirror" is to maintain an exact copy of something. Probably the most common use of the term on the Internet refers to "mirror sites" which are web sites, or FTP sites that maintain copies of material originated at another location, usually in order to provide more widespread access to the resource. For example, one site might create a library of software, and 5 other sites might maintain mirrors of that library. From Matisse

2. Keeps FTP archives up-to-date Mirror uses the FTP protocol to locally duplicate remote host files and directories selected with Perl regular expressions. By default transfers only files missing locally or whose remote sizes or time-stamps have changed. Can reduce directory download using compressed listings in ls-lR.gz files or further using compressed differences of daily listings in ls-lR.patch.gz files. Amongst many flexible options it can gzip and split files. Tracks large distant FTP archives accurately with low download volume. Simpler programs like "mirrordir" use less memory and may copy directory trees faster between local machines. From mirror.tar.gz 2.9 in Perl by Lee McLoughlin. From Debian 3.0r0 APT