The failover configuration requires two identical security appliances connected to each other through a dedicated failover link and, optionally, a Stateful Failover link. The health of the active interfaces and units is monitored to determine if specific failover conditions are met. If those conditions are met, failover occurs.
The security appliance supports two failover configurations, Active/Active failover and Active/Standby failover. Each failover configuration has its own method for determining and performing failover.
With Active/Active failover, both units can pass network traffic. This lets you configure load balancing on your network. Active/Active failover is only available on units running in multiple context mode.
With Active/Standby failover, only one unit passes traffic while the other unit waits in a standby state. Active/Standby failover is available on units running in either single or multiple context mode.
Both failover configurations support stateful or stateless (regular) failover.
http://www.cisco.com/univercd/cc/td/doc/product/multisec/asa_sw/v_7_1/conf_gd/failover.htm#wp1042999
