SQL Server Availability Groups and Distributed AGs

Let's take a look around the magic that is SQL Server Availability Groups! What are they? What are they used for? And what is a 'distributed' availability group?!?! This is a fairly high level session, but assumes general knowledge of MS SQL Server, SSMS, and TSQL. Anyone who might be new to availability groups, or just curious, or would like to explore Distributed Availability Groups functionality, would enjoy this one.