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 websites, or FTP sites that maintain exact copies of material originating at another location, usually to provide more widespread access to the resource. Another common use of the term 'mirror' refers to an arrangement where information is written to more than one hard disk simultaneously so that if one disk fails, the computer keeps on working without losing anything.