Maduro means ripe. On a cigar band it usually means a dark brown to near-black wrapper, taken further in fermentation (and sometimes in time and heat) than a natural leaf from the same crop. It is a process name more than a seed name.
Darker is not automatically stronger. A maduro wrapper can be sweet, earthy, or quiet; the filler still sets most of the strength. Houses such as Padrón bottle the same blend in natural and maduro wrappers so you can taste what the extra fermentation does to the cover leaf.
Not every dark wrapper is a maduro in the factory sense, and not every maduro is oscuro. The words overlap in shops. This page uses maduro for a deliberately darkened, longer-fermented wrapper.