SQL Server Licensing & Clustering for Availability Groups
In SQL Server 2012 and 2014, the Availability Groups feature requires an underlying WSFC (see this post for info on newer versions). You can use FCIs in combination, but it is not required (and in fact often overkill, depending on what you are trying to accomplish and whether you have underlying redundancy as well).
WSFC is the underlying Windows Server Failover Cluster. FCI is just the terminology for a single SQL Server failover cluster instance. An FCI depends on a WSFC, and can use 1-n nodes of the WSFC to provide failover scenarios for the clustered instance. Some good background here from Allan Hirt. Basically, an FCI protects a single, entire instance of SQL Server (when a failover happens, the entire instance moves to the next preferred owner in the WSFC). A WSFC protects an instance of Windows, and any SQL Server instances on it. An Availability Group uses the underlying WSFC to protect a group of databases.
In 2012 / 2014, Standard Edition does not support Availability Groups in any shape or form - Enterprise Edition is required. In SQL Server 2016 and above, there is "Basic Availability Group" for Standard Edition - 2 nodes only, 1 database per group, no formal listener support, no read-only secondaries, etc. Official Documentation...