What Are SQL Server AlwaysOn Availability Groups?
AlwaysOn Availability Groups are a high-availability and disaster-recovery feature introduced with SQL Server 2012, providing enhancements and abilities over database mirroring. An Availability Group protects a set of user databases, supporting up to eight secondary replicas of the primary database. In the event of a failure on the primary, all of the databases within an Availability Group will fail over together and become active on the designated failover partner. Additionally, you can configure secondary replicas to allow read-only activity and most backup operations, potentially taking some of the load away from the primary.