Prior to SQL Server 2012, SQL Server had several high availability and disaster recovery solutions for an enterprise’s mission critical databases such as failover clustering, database mirroring, log shipping or combinations of these. Each solution typically has a major limitation, in the case of failover clustering for example, its configuration is very tedious and complex and you arguably have single shared storage or single point of failure. Database mirroring is relatively easy to configure in comparison with failover clustering, but you can have only one database in a single mirroring setup and you cannot read from the mirrored database. Log shipping does not provide automatic failover (higher availability) though it be used for disaster recovery with some expected data loss.
SQL Server 2012 (Code name SQL Server Denali) introduces a new feature called AlwaysOn which combines the best of failover clustering and database mirroring and overcomes major of the limitations imposed in failover clustering or a database mirroring setup, for information click here.