As we all know, the Legion is on it's way. At this Sunday's meeting in the new Stormwind Park we'll talk a bit more about what we know and what we can expect over the next month.
The best source of information is Legion Preview Events
At some point in early August (possibly as early as Aug 2nd after reset) we will start experiencing invasions on the continent of Azeroth. Why is this important to know? Because these events will only last until the expansion launches on Aug 30th. So the titles, items and pets you can get from the Invasion event are only attainable during this small window of time. Because of this, all regular [GSCH] calendar events are on hold from Aug 2 until Sept 6. Retro raids, dungeons and battlegrounds will always be in the game, but during the Legion invasion I will be spending my online time coordinating groups to hit as many Legion invasions as possible. They are scenerios and we will be fighting the opposite faction in each scenerio as well as working through the four stages of the invasion. Ultimately, the more you know going in the better your chances of success :)
Here is a sample of two Legion invasion events from the PTR. Don't watch if you'd like to be surprised!
Invasion Stages
There are six zones you can encounter an invasion in. Invasions will be available in the Legion prepatch and do not require players to have completed Broken Shore questlines.
Completing one invasion rewards 
Stand Against the Legion and all six rewards Defender of Azeroth: Legion Invasions.
