This document discusses scaling applications horizontally on AWS. It provides an overview of AWS services like EC2, load balancers, auto-scaling groups, and databases that can be used to build scalable architectures in the cloud. Key recommendations include making applications stateless, using distributed caching, executing jobs only once in a cluster, and automating infrastructure provisioning with tools like CloudFormation. The document also covers blue-green deployments and general best practices for security and availability.