Use Amazon RDS for transactional applications needing

Pre-defined schema

Strong transactional capabilities

Complex queries

Amazon RDS is NOT recommended when

You need highly scalable massive read/write operations - for example

millions of writes/second

Go for DynamoDB

When you want to upload files using simple GET/PUT REST API

Go for Amazon S3

When you need heavy customizations for your database or need access to

underlying EC2 instances

Go for a custom database installation