This is an interesting post on the different types of AWS keys and login credentials you'll need with AWS:
http://alestic.com/2009/11/ec2-credentials
Only thing I'd also add is the use of IAM roles which are basically set up so you don't have to store credentials on a machine running AWS services. The services can make use of the role which is built into AWS EC2 instances when running on an EC2 instance -- that way you don't need to store the credentials anywhere on the machine:
http://docs.aws.amazon.com/AWSSdkDocsJava/latest/DeveloperGuide/java-dg-roles.html
