There are tons of preschool philosophies and options available today. Three common philosophies today are Montessori, Waldorf and Reggio (all are named after the theorist who founded them).
Montessori is self-paced education with the teachers as guides, they prepare the environment and observe based on the assumption that the child is auto-educated (think self driven). It focuses on providing materials that foster children's self-directed learning and ample time for children to explore the center. It is a fairly structured preschool setting.