There seems to be an increasing overlapping and proliferation of cloud database technologies.
In order to make sense of it a comparative approach might help.
What are the exact differences between Google Cloud Firestore vs Google Cloud Spanner ?
Cloud Firestore is:
A flexible, NoSQL (non-relational) scalable database for mobile, web, and server development from Firebase and Google Cloud Platform.
On the other hand, Cloud Spanner:
Horizontally scalable, strongly consistent, relational database service.
So the main difference between them is that one is a non-relational database while the other is relational. Furthermore, Cloud Firestore is also a real-time database, which means that for every change that takes place in the database you are instantly notified.