A failover group is simply a logical group of one or more security contexts. In Active/Active failover, you divide the security contexts on the ASA into 2 failover groups. Active/Active failover is only available to ASAs in multiple context mode. In an Active/Active failover configuration, both ASAs can pass network traffic.
You can use Active/Standby failover for ASAs in single or multiple context mode.
When a failover occurs, the active unit fails over to the standby unit, which then becomes active. The standby unit does not actively pass traffic.