@Sitar_Ned, Here is a short one
A gravel bike, is an endurance race bike( more relaxed position than a pure race bike ), usually with disc brakes and that allows big tires( ~40mm ).
And it is all the rage in the US... well it seems from here in Europe.
I'm planning to build a road bike early next year. But due to my long legs, the gravel geometry is perfect for me.
However I just commented to Peter, that this one wasn't that relaxed as I was expecting.