One of the most popular type of preschools and usually the most loosely defined, play-based preschools do have structure, but the focus is on play and socialization, rather than academics. That isn't to say that the children don't learn, but lessons are generally done through play-based activities and have their roots in basic social skills, like helping children to get along, learning to share and taking turns.
Critics say that play-based preschools don't provide children with enough academic knowledge to do well in kindergarten, but those who support this method say that young children, while often good at memorization, don't have the cognitive resources and development yet to apply facts they can recite. For example, just because a 4-year-old child can count to 20, they cannot always figure out the answer to 3 + 1. Proponents also say that it's socialization that is most needed at this young age.
These school can also be referred to as social-based preschools.