High Availability, or HA, means designing systems to stay available even when some components fail. In cloud computing, this often uses multiple instances, zones, load balancers, health checks, and failover.
Why HA matters
Users expect important applications to keep working. HA reduces the chance that one server, zone, or component failure will interrupt the entire service.
HA is not the same as disaster recovery
HA focuses on continuous availability. Disaster recovery focuses on restoring service after a larger failure or outage.
HA is about removing single points of failure wherever the business needs reliability.