IAM General

IAM Dashboard

IAM is universal. It does not apply to regions at this time.
The "root account" is simply the account created when you first setup your AWS Account. It has complete Admin access.
Power User Access allows access to all AWS services except the management of groups and users within IAM.

Roles

Roles are more secure than storing your access key and secret access key on individual EC2 instances.
Roles are easier to manage.
Roles can be assigned to an EC2 instance after it is created using both the console & command line.
Roles are universal - you can use them in any region.