Butting is a term thrown around a lot in frames, forks, bars, spokes, etc. Anything with tubing, really. But what is butted tubing? Well, simple the thickness of the metals varries, instead of being constant.
External butting is also used in frames. For example, look at the Mankind Archangel frame:
External butting is also popularly used in spokes.
In the technical drawing above, both tubes are butted. It may be confusing to think about, when you hear "double butting", you think "okay, there are 2 steps down in thickness." however, it's much different. Still simple, though
Butted: The tube varies in thickness at one end
Double Butted: The tube is thicker at the ends, and both ends are the same thickness
Triple Butted: The tube is thicker at the ends, but the ends are of varying thickness
Quad. Butted: The ends and center are of varying thickness.
Straight Gauge: No change in thickness. .