-Non butted spokes have the same wire diameter for the entire length of the spoke.
-Double butted have the same wire diameter at the ends with a smaller diameter wire in the middle.
-Triple butted spokes have the largest diameter wire at the hub end, smallest diameter wire in the middle, and yet another wire diameter for the nipple.

Butting, to my understanding, allows the spoke to better transition from supporting the weight of the bicycle to the tension it was built to when not supporting the bicycle due to elasticity of the butted spoke. The (usually) 1.8 mm diameter of the center section of a DB spoke is plenty strong for touring purposes and is quite common although an argument can be made that a 1.8 mm non butted spoke isn't quite as strong as a 2.0 mm non butted spoke.

HTH,

Brad

PS HTupolev and I must have been typing an answer at nearly the same time, his is more detailed!