Montessori is a method of education that is based on the observations of Dr. Maria Montessori. At its foundation is the recognition of each child as an individual. Attention is given to all aspects of a child’s development—social, emotional, physical, and cognitive. Multi-aged classrooms, equipped with specially designed materials, provide a structured environment in which children have freedom of movement and choice, enabling them to become independent, confident learners.
American Montessori Society
The American Montessori Society is the largest Montessori organization in the world. The "What is Montessori" section of their site provides useful history and insight on the Montessori philosophy.
North American Montessori Teacher’s Association
The NAMTA site also provides an "About Montessori" link that has good overviews of Montessori tools and terminology, and addresses popular misconceptions.
Montessori Today: A Comprehensive Approach to Education from birth to Adulthood, by Paula Polk Lillard (Buy it at Amazon.com)
Montessori: A Modern Approach, by Paula Polk Lillard (Buy it at Amazon.com)