Before I start, I will talk about the terms used here – these are standard terms used in many of Microsoft’s documents and on this site:
FCI – Failover Cluster Instance. This is where SQL Server is installed as a clustered instance.
WSFC – Windows Server Failover Cluster. The core component required to be able to implement Always-On Availability Groups. All nodes must be part of the same WSFC and you enable the Availability group feature from within SQL Server Configuration Manager.
AG – Always-On Availability Group. The way to setup “mirroring” of databases with SQL Server since Database Mirroring has become deprecated (to be removed from a future release of SQL server). AG is the common and proper acronym for Availability Groups.
I will detail how to setup an FCI on a WSFC using Availability Groups. You can even do this with SQL Server standard edition giving you High Availability and Disaster Recovery.