AWS IAM Role vs Group

AWS Groups are the standard groups which you can consider as collection of several users and a user can belong to multiple groups.

AWS IAM Roles are all together different species; they operate like individual users except that they work mostly towards the impersonation style and perform communication with AWS API calls without specifying the credentials.

Given that IAM Roles are little different, I am emphasizing only that. There are several types of IAM Roles like EC2 IAM Roles, Lambda etc. If you consider, you can launch an EC2 instance with an EC2 IAM Role; hence forth any AWS API related communication wouldn't require any AWS Access Key or Secret key for authentication rather can call the APIs directly (however the long answer is - it uses STS and continuously recycles the credentials behind the scenes); the privileges or permissions of what it can do is determined by the IAM Policies attached to the IAM Role.

Lambda IAM Role works exactly the same, except that only Lambda function can use the Lambda IAM Role etc.

Short answer for googlers: you can't assign role to user.

• group is a bunch of users with the same policies
• role is a preset of policies for service(s)

Users can asume roles according to AWS docs:

Assuming a Role

Users: End User (Think People).

Groups: A collection of users under one set of permissions (permission as policy). As per IAM standards we create groups with permissions and then assign user to that group.

Role: you create roles and assign them to AWS resource (AWS resource example can be a customer, supplier, contractor, employee, an EC2 instance, some external application outside AWS) but remember you can't assign role to user.

It’s not only users who will login, sometimes applications need access to AWS resources. For example, an EC2 instance might need to access one or more S3 buckets. Then, an IAM role needs to be created and attached to the EC2 instance. That role can be re-used by different EC2 instances.

Remember : Groups are for living. Roles are for non-living.