AWS Security Groups Guide
AWS Security Groups (and Network ACLs and VPCs) are some of the fundamental building blocks of security in your cloud environment. They are similar to firewalls but are not the same thing. You have to understand this topic very well before you begin building in the cloud, because there are some subtle differences in how they are used, and you need to follow best practices. You should know your public cloud provider is contractually bound to honor its side of a shared responsibility model.