HA features are meant to be implemented for Mission Critical Activities... In simple terms it implies implementing redundancy into your servers in means of its Power Supply, Memory, Hard Disk, Even CPU. The aim is very simple To eliminate Single Point of Failure for mission critical activities

The High availability solution can be implemented using technologies like clustering , mirroring, SAN replication, Hot swap features for many hardware devices, Pre-Failure alarms for early detection of problems, making OS failover proof etc.

I hope this will give you some idea on the concept...