In a VPC, a subnet is a range of IP addresses where resources can be isolated and managed. Having multiple subnets allows a company to separate different tiers of their application (such as web servers in a public subnet and databases in a private subnet) to increase security and manage traffic flow within the VPC. The Availability Zone is a larger concept where AWS infrastructure is organized, and gateways are for allowing traffic in and out of the VPC, not for isolation within it. While security groups are used for setting access controls to resources, they do not isolate network parts.