The wrapper is called "capa" in Spanish, which translates more accurately to cloak or cape. The function of the wrapper is simply that a cape or cloak- and think of the wrapper as a nothing more than a cape wrapped around the cigar.
The wrapper is a special leaf. Smaller almost insignificant veins, oily silk-like texture, and an elastic anatomy are the qualities of a good wrapper. To achieve this, nearly every aspect of the of the agricultural and production process is very different from that of the filler.
There are several variations of color based changing certain elements in the process. The most common variations are listed here.
Why do I say that the wrapper is misunderstood... Just like an idiot in a nice jacket, what is on the inside it what makes the cigar. Do not be mislead, maduro or oscuro wrapper are not stronger. Oscuro wrapper may leave a stronger taste in your mouth, but that is coming from the unsmoked tobacco and isn't a good sign at all.
So in short, do not be mislead, and do not be tricked, the wrapper does not make a cigar stronger, nor does it make it better (if it is garbage on the inside). Don't get me wrong, a beautiful wrapper is a great way respect a great cigar, but my advice is- Don't get too caught up with the wrapper.
A pampered natural wrapper is what I prefer, slightly dark, and a little shiny, although the best cigar I ever smoked actually had no wrapper at all. It's binder was prepared and humidified like a wrapper. It left a raisiny flavor in my mouth, and I loved every puff.
So don't get caught up, and Enjoy!