Both Realtime Database and Cloud Firestore are NoSQL Databases.

Realtime Database	Cloud Firestore
Stores data as one large JSON tree. Simple data is very easy to store. Complex, hierarchical data is harder to organize at scale. Learn more about the Realtime Database data model.	Stores data as collections of documents. Simple data is easy to store in documents, which are very similar to JSON. Complex, hierarchical data is easier to organize at scale, using subcollections within documents. Requires less denormalization and data flattening. Learn more about the Cloud Firestore data model.