Google Cloud Functions vs Cloud Functions for Firebase

Google Cloud Functions

https://cloud.google.com/functions/

• This is the primary serverless product on Google Cloud Platform
• Currently support Node.js, Python and Go Runtime
• Support event by Cloud Pub/Sub, Cloud Storage, Stackdriver logging, Firebase, Direct HTTP, etc.

Cloud Functions for Firebase

https://firebase.google.com/docs/functions/

• This is like a wrapper around Google Cloud Functions, with more seamless support/intergration with Firebase
• All Cloud Functions listed in Google Cloud Platform Console shall appear in Firebase Console as well
• Support event by Firestore, Realtime Database, Remote Config, Firebase Authentication, Google Analytics for Firebase, Crashlytics, Cloud Storage, Cloud Pub/Sub, HTTP, etc.
• Seems to support JavaScript and TypeScript only.

Cloud Functions for Firebase client SDK

https://firebase.google.com/docs/functions/callable

• Let you call functions directly from a Firebase app (so you don't have to write http call/retrofit code)
• Supported client: iOS, Web, Android, C++, Unity
• Officially, you should call Cloud Functions for Firebase. But, you can make Google Cloud Functions compatible for Firebase.

References:

• https://cloud.google.com/functions/docs/concepts/functions-and-firebase
• https://stackoverflow.com/questions/42854865/what-is-the-difference-between-cloud-functions-and-firebase-functions
• https://medium.com/google-developers/firebase-google-cloud-whats-different-with-cloud-functions-612d9e1e89cb