IAM Roles allow trusted Users, AWS services and applications to adopt a set of temporary IAM credentials to access your AWS resources. Roles act as identities, much like Users do, and have permissions assigned to them defining what resources the Roles can and can't access. Unlike Users though, which represent a single identity, IAM Roles are designed to be assumed by multiple different entities as and when required. Like I say, Roles are used for temporary access to gain access to resources, and each time the role is assumed by a User, an AWS service or an application, a new set of credentials is dynamically created for the duration of that session. As a result, Roles do not have any long term credentials associated, so there is no password for console access, nor are there any access keys for programmatic access that are explicitly associated with the Role.

For every role there will be associated policies controlling access as to what can and can't be accessed when the role is assumed. There will also be a Trust Relationship, and this Trust Relationship defined who or what can assume the role, for example a User, an AWS account, or an AWS Service. IAM Roles are generally used: if you need to grant temporary access for Users to AWS resources that they don't normally require access to or you can use a Role to grant access for an IAM user in one account to access resources in another AWS account or perhaps an AWS service needs to access resources on your behalf or if an application requires access to resources you can use a Role instead of embedding credentials into the software itself. Or you might have federated users who require access to specific resources, perhaps those authenticated via Active Directory So, as a result, Roles can be assumed by the following: A user that's in the same AWS account as the where the role has been created, a user that's in a different AWS account than where the the role has been created, an AWS service, such as EC2, or an external federated users to your AWS account