Overview
Cognito is composed of two things:
- User pool = managed OAuth server
- Identity pool = connect IAM with other identity services
User Pool
- User pool = the managed OAuth server that manages your users
- User pool client = when you register a new user, or let a user sign in, you need to call the OAuth server without authentication, to do that, you need to create a client and get client ID
- Most of the time this is how your backend communicates with user pool
- User pool domain = a hosted login / signup page for your users in case you don’t want to create your own
The AWS Blog 