AWS Certified Solutions Architect Associate Test 1

Use roles for applications that run on Amazon EC2 instances.
Applications that run on an Amazon EC2 instance need credentials in order to access other AWS services. To provide credentials to the application in a secure way, use IAM roles. A role is an entity that has its own set of permissions, but that isn't a user or group. Roles also don't have their own permanent set of credentials the way IAM users do. In the case of Amazon EC2, IAM dynamically provides temporary credentials to the EC2 instance, and these credentials are automatically rotated for you.
http://docs.aws.amazon.com/IAM/latest/UserGuide/best-practices.html#use-roles-with-ec2