Blizzard is hoping to cater to social groups by providing a raid difficulty level that caters to varying numbers of players.
The Flexible Raid System is designed to slot in between Raid Finder and Normal difficulty, and is open to pre-made groups of ten to 25.
What that means is that you and you guildies, friends, colleagues and family don't have to be really strict about how many players you have, and can just group up and have a good time without having to hang out with strangers.
There are no Item Level requirements, but there's also no matchmaking. The challenge of the raids will vary depending on how many people you bring.
Loot drops will be slightly better than in Raid Finder but not as good as Normal, and will use the same per person system as Raid Finder.
Blizzard intends to roll out the new difficulty in stages, more quickly than it did with the Raid Finder. For more information on the new system, head on over to Battle.net.