Introduction to Cisco Firewall Failover

Failover Analogy

"Even the greatest warriors in the world have a moment of weakness. In football there is always a guy sitting on the bench just waiting for the quarterback to break an arm for a shot at some fame. The ASA has a similar functionality of this using failover. With failover enabled (2 ASAs required) you can have a active and a standby unit or both units can be active (multi context). In Active/Standby mode one ASA is out performing the dangerous job of being a firewall while the standby is just waiting to come into action when its good active ASA buddy goes down."
http://www.wr-mem.com/?p=110

Failover definition  

"Failover is the capability to switch over automatically to a redundant or standby computer server, system, or network upon the failure or abnormal termination of the previously active server, system, or network. Failover happens without human intervention and generally without warning, unlike switchover.
Systems designers usually provide failover capability in servers, systems or networks requiring continuous availability and a high degree of reliability.
In some cases, computer system failover is not desired to be automatic, but is required to allow human intervention to effect the failover. This is called "automated with manual approval", as the activity is automatic once approval is given."
http://dictionary.sensagent.com/failover/en-en/
http://en.wikipedia.org/wiki/Failover