Retention is the process of bringing users back to the digital platforms be it a website, social media platform, etc. 

User retention focuses on retaining your existing users. Understanding retention can help you assess your current users if they are happy with your product and services or not. 

On the other hand, engagement is the process of interaction between the brands and their audiences both online and offline. 

To know more about user engagement and its crucial KPIs, read this detailed guide.