What is deleted: your login and your profile. Families where you are the only member are deleted with all of their memories, photos, logs and comments.
What stays: families that other people belong to are never deleted. You leave them; if you were the only owner, ownership passes to the next parent or caregiver. Memories you added there stay with the family without your name.
Subscriptions: deleting your account does not cancel a Google Play subscription. Cancel it first under Google Play → Payments & subscriptions. A plan you paid for stays with the family until the paid period ends.
Deletion is immediate and permanent. Photos are removed from storage right away.