Trusted Advisor covers four main areas and it is designed to give you some guidance around what are considered best practices for your cloud deployment. The areas covered are:

• Cost Optimization
• Performance
• Security
• Fault Tolerance

It's available to everyone and free to use—with one fairly large catch. Unless you are paying for Business or Enterprise level support with AWS you only get access to four checks. At the time of publishing there are 55 possible checks.