HA/DR refers to High Availability / Disaster Recovery, which is typically a suite of technologies provided by a database engine vendor with the aim of increasing availability of data stored in the database, and providing a fast method of return-to-service after a disaster.

DB2® High Availability Disaster Recovery (HADR) uses database logs to replicate data from the primary database to the standby database. Non-logged operations are allowed on the primary database, but not replicated to the standby database. If you want non-logged operations, such as updates to the history file, to be reflected in the standby database, you must take extra steps to cause this to happen.

Microsoft SQL Server 2012 HA/DR is referred to as AlwaysOn. The AlwaysOn Availability Groups feature is a high-availability and disaster-recovery solution that provides an enterprise-level alternative to database mirroring. An availability group supports a failover environment for a discrete set of user databases, known as availability databases, that fail over together. An availability group supports a set of read-write primary databases and one to four sets of corresponding secondary databases. Optionally, secondary databases can be made available for read-only access and/or some backup operations.