Cisco ASA – Difference Between Cluster and Active/Active Failover

The active/active cluster allows you to split traffic into groups so the A node handles traffic for some networks while the B node handles traffic for the others, in the event of a failover the stable unit will handle traffic for both A&B traffic groups (assigned on a per context basis). Active/Standby cluster handle all traffic on a single node while the other is brought into use only in the event of a failure.

This is generally used when you're going to have more traffic than a single node can handle, if one of the units fails, instead of stopping all traffic destined to the failed ASA it will run in a degraded state until it is replaced. So essentially you're stretching you're hardware and actually utilizing it taking the risk of degradation if there's a failure instead of paying for a standby unit that essentially never sees any traffic pass through.

Edit: Forgot to clarify active/active is only available for multi-context mode since it would serve no purpose otherwise since you couldn't create the groups