There are a number of outstanding sources of information about monarch biology.
Journey North engages students in a global study of wildlife migration and seasonal change. The site has a Frequently Asked Questions section with a wealth of information about monarch biology at http://www.learner.org/jnorth/search/Monarch.html.
The Monarch Butterfly in North America
From the USDA Forest Service
This web site has basic information about the monarch life cycle, migration and overwintering, and habitat needs.
Monarch Lab: Monarchs in the Classrooms
Monarch Lab has several sections with information about monarch biology including:
-- Annual Life Cycle
-- Monarch Life Cycle
-- Global Distribution
-- Ask the Expert
Monarch Watch has information on anatomy, sexing monarchs, life cycle, sensory systems, predation, and more.
North American Monarch Conservation Plan
The North American Monarch Conservation Plan has an excellent fact sheet and description of the species.