Final Fantasy 14's Stormblood expansion is poised to bring with it an entire game's worth of new content, a level cap increase, new raids and dungeons, a reworked battle and inventory system, and the list goes on for quite a while. But perhaps most importantly of all, at least as far as gameplay is concerned, are the new and extremely flashy jobs (read classes): Red Mage and Samurai.
In order to prepare everyone for what is to come the developers have released the preliminary patch notes for Stormblood... and they are ridiculously massive! I would usually go through all of them and pick out a couple to showcase the update's direction, but there are simply far too many changes to even attempt such a thing. As such, if you're interested in all of the details and you have plenty of coffee at hand, you'll find what you seek at the official website.
On the other hand, if you value your sanity and would like to get a quick overview of what Stormblood brings to the table, you'll find the recently posted launch trailer right below. Its not very detailed, but its at the very least interesting and quite pretty to look at. Here it is:
As far as release dates are concerned, you'll be able to explore all of the new content on June 20th. The most dedicated among you, however, will be able to begin their adventures a couple of days early, on June 16th when the Early Access phase begins. To learn more I would suggest going to the Stormblood website.