Bendy bones? What's that? Well, it's a HUGE improvement on B-Bones.
You still don't know what that is? Let me explain: basically, you create a bone, but you want it to bend (without adding extra bones) by breaking it in pieces so it reacts like some kind of bezier curve. Up to this point we had B-Bones, which kind of used to do this, but there were some issues, not many options to customize their behavior... but look a this:
Follow the link above to find more information about these improvements and what the different options of the bendy bones do.
Basically, these new B-Bones allow for a wide range of setup for animation purposes, which before would have to be created with a chain of actual bones following a curve. Now it's possible to quickly and easily define the curvature and divisions of a bone, thanks to these "bendy bones". This feature is coming in Blender 2.78, which is about to come out :D