You will likely get a variety of answers about what teachers call the different times of day (and what they call their centers/interest areas/learning centers). A Preschool classroom is typically set up into centers/interest areas such as blocks, art, pretend play (sometimes called dramatic play, house corner, housekeeping), sand and water, discovery (science), music, library/book area (which may also include an area for writing), and an area for toys and games such as math manipulatives. When these areas (or centers) are open for free play the children are selecting the materials (that the teacher has thoughtfully provided in each area). At other times of day, the teacher works with children in small groups within one of the areas on a specific activity from their lesson plan- such as a sorting activity, a shape matching games, or a letter recognition game.
Children are learning through play and there is plenty of research to back this up. The teacher facilitates learning by setting up the environment, providing the materials and opportunities for exploration, and using effective strategies for guiding children's learning.