Failover
A method of ensuring that if a server crashes (stops working), another one will start up and assume its duties, with minimal or no visible interruption to the end-user. Users often want failover monitoring to ensure their systems are always monitored.

Firewall
Either software or hardware (e.g. a network device) that protects a network and a computer system on the network from outside attacks and connection attempts. Many routers include a firewall.

Flow
Flow is a sequence of packets from a source device to a destination device.