UWO is just the power generated (in watts) when the drill or whatever is going full tilt. Torque is a measure of the drills ability to turn an object, like a screw.
Using similar units, UWO is in Nm/s (Newton-metres per second) and torque is in Nm (Newton-metres). The reasoning from Dewalt is that looking at torque over time gives a more accurate description of a drill's potential than peak torque (which is, I think, what most torque listings tend to be; thank you Surge marketing materials).
This reasoning makes sense to me because you run a drill over a period of time rather than for an instant. Any engineers on the board might have a more informed opinion than I do about this.