Here's the article for ZMA. It does a good job explaining the three ingrediants. I know, I know, it's from wiki, but I trust that more than a supplement site's description, since most of the time they try to make their product sound better than it really is.
http://en.wikipedia.org/wiki/ZMA_%28supplement%29
And here's, in my opinion, a slightly more exaggerated description of the supplement, but essentially the same information.
http://www.bodybuilding.com/fun/top-...u-need-zma.htm