Montessori is a comprehensive educational approach from birth to adulthood based on the observation of children’s needs in a variety of cultures all around the world.
Beginning her work almost a century ago, Dr. 
Maria Montessori developed this educational approach based on her understanding of children’s natural learning tendencies as they unfold in “prepared environments” for multi-age groups (0-3 year olds, 3-6 year olds, 6-9 year olds, 9-12 year olds, and 12-14 year olds).
The Montessori environment contains specially designed, manipulative “materials for development” that invite children to engage in learning activities of their own individual choice. Under the guidance of a trained teacher, children in a Montessori classroom learn by making discoveries with the materials, cultivating concentration, motivation, self-discipline, and a love of learning.
Today, Montessori schools are found worldwide, serving children from birth through adolescence. In the United States, there are more than 4,000 private Montessori schools and more than 200 public schools with Montessori-styled programs.
(Quoted from the NAMTA site)