Cisco IOS (Internetwork Operating System) is a proprietary operating system that runs on Cisco Systems routers and switches. The core function of Cisco IOS is to enable data communications between network nodes.
In addition to routing and switching, Cisco IOS offers dozens of additional services that an administrator can use to improve the performance and security of network traffic. Such services include encryption, authentication, firewall capabilities, policy enforcement, deep packet inspection, Quality of Service (QoS), intelligent routing and proxy capability. In Cisco's Integrated Services Routers (ISRs), IOS can also support call processing and unified communications services.
Cisco IOS software releases are organized into what Cisco calls "families" and "trains." Each family shares the same code base and trains are how new IOS releases are delivered.
There are two types of IOS operating systems:
IOS XE - runs on top of a Linux kernel. IOS XE and IOS share a lot of the same code, but IOS XR is considered to be a completely different code base.
IOS XR - based on QNX a commercial Unix-like real-time operating system. IOS XR supports software-defined networking (SDN) and the embedded systems market.