IOS is the operating system of cisco routers and switches. The name is a acronym for Internetworking Operating System. Usually you access it by telnet.  It is unix based operating system and everyone that know unix feels at home when telnetting into cisco router. Good resourse about the cisco IOS commands is:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/12cgcr/rbkixol.htm