My understanding is that if one wants to implement Always On / Availability Groups (without FCI) the Windows Clustering Service would need to be installed, i.e. a Windows Cluster is required but not a SQL Cluster. Is that correct?
MSDN has an excellent article about this exact topic.
Some salient points:
Ensure that each computer is a node in a Windows Server Failover Clustering (WSFC) cluster.
SQL Server Clustering can be used, but is not required.
From this MSDN page, we get:
AlwaysOn Availability Groups, the high availability and disaster recovery solution introduced in SQL Server 2012, requires Windows Server Failover Clustering (WSFC). Also, though AlwaysOn Availability Groups is not dependent upon SQL Server Failover Clustering, you can use a failover clustering instance (FCI) to host an availability replica for an availability group.