Oh! The biggest change is probably that most raiding, except the highest level, will be flexible sizes. This is like the current "Flex" tier of Siege of Orgrimmar, where you can take 10-25 people to the raid, and the content will scale. You can even lose/gain people as the night goes on. This should make scheduling wayyy easier, and, outside of class balance (e.g. still need 2 tanks), will allow more raid groups to combine and split more deftly.
"Mythic" is the highest level of raiding, which replaces current Heroic. It's the only fixed-size raid difficulty, with 20 players required.
With the new flex style, you don't even have to compete for raid spots!