Which service is primarily used for user management in mobile and web applications?

Amazon Cognito is primarily used for user management in mobile and web applications because it provides a fully managed service that simplifies the process of adding user sign-up, sign-in, and access control to applications. It efficiently handles user authentication and authorization, enables social login through providers like Google and Facebook, and allows users to maintain their profiles in a secure manner. Additionally, Cognito supports scalable user directories and can integrate with other AWS services for access control.

In contrast, while AWS IAM (Identity and Access Management) is used for managing permissions and access to AWS resources, it is not designed specifically for application user management. AWS Lambda, being a serverless compute service, is focused on running code in response to events rather than managing users or authentication. Amazon RDS (Relational Database Service) is a database management service and does not provide capabilities for user management in applications. Thus, Amazon Cognito stands out as the best option for managing users in mobile and web applications.