"Agenda 21 is a comprehensive plan of action to be taken globally, nationally, and locally by organizations of the United Nations System, Governments, and Major Groups in every area which humans impacts on the evironment."
-- United Nations Publication
Agenda 21 Publication (from the official U.N. Website)
For a quick synopsis, you can check out the following video clip "Agenda 21 for dummies:"
This is less than 10 minutes long.