High Availability - AlwaysOn Failover Clustering

Overview

Failover Clustering (or AlwaysOn Failover Clustering) is a means for providing redundancy at a server level. Should one server fail, another server will take it's place automatically. Clustering requires two (or more) servers, and a shared disk. Standard Edition will only support a 2 node failover cluster.

Pros

Automatic failover from one server to another
Appears as one SQL Server on the network
Redundancy at a server level
Robust
AlwaysOn supports failover cluster nodes in different geographical locations (via virtual LAN)

Cons

Expensive, more involved (tricky) to set up and configure than other High Availability methods, can be sluggish to failover. Not all SQL features are able to be clustered (i.e. Reporting Services). Data is only in ONE place (i.e. on a shared disk) - use RAID / Disk Mirroring to mitigate this risk

Clustering resources

Really great post here (by Kendra Little on Brent Ozar blog): http://www.brentozar.com/archive/2012/02/introduction-sql-server-clusters/
MSDN: https://technet.microsoft.com/en-us/library/ms189134(v=sql.110).aspx