play group vs preschool

play group and preschool both are nouns.

Word	Noun	Adjective	Verb	Adverb
play group	Yes	No	No	No
preschool	Yes	No	No	No

As nouns, preschool is a hypernym of play group; that is, preschool is a word with a broader meaning than play group:

play group: a small informal nursery group meeting for half-day sessions
preschool: an educational institution for children too young for elementary school

play group (noun)	preschool (noun)
a small informal nursery group meeting for half-day sessions	an educational institution for children too young for elementary school