Change #1: Beginning in the Fall of 2015, Magic Blocks Will Be Two Sets Each
Starting with "Blood" block in September 2015, each block will have one large set that introduces the world and one small set that builds upon it. (I should note that this is the default and we reserve the right to have sets of different sizes if it serves the block.) Blocks will no longer have a third set.
Change #2: Beginning in the Fall of 2015, Magic Will Have Two Blocks Per Year
Here's the typical schedule of the Magic year, with all the usual caveats about how Magic is about change and you should expect us to shake this up a bit over time: There will be a large fall expansion and small winter expansion set in World #1. Then there will be a large spring expansion and a small summer expansion, usually set in World #2. The first two sets will be part of one block and the second two sets will usually be part of a second.
Change #3: Beginning in 2016, the Core Sets Are No More
The summer of 2015 will have the last core set release, and in 2016, the summer set will be the second set of the block that started in spring of 2016.
Change #4: Beginning in 2016, the First Set of Each Block (the Fall and Spring Sets) Will Cause a Rotation
This in some ways isn't a change, but rather an adaptation to match the new Two-Block Paradigm. The first set of a block has always caused a rotation in the past. The Two-Block Paradigm just means that this now happens twice a year (in the fall and in the spring) rather than just in the fall.
Change #5: Beginning in 2016, Standard Will Be Three Blocks Rather Than Two Blocks
To keep the metagame more robust, we are changing the size of Standard. This change is the reason we are telling you this now, because we want you to understand that Khans of Tarkir will only be played in Standard for 18 months. This change should not have any impact on the set's role in other formats.