security group

A named set of allowed inbound network connections for an instance. (Security groups in Amazon VPC also include support for outbound connections.) Each security group consists of a list of protocols, ports, and IP address ranges. A security group can apply to multiple instances, and multiple groups can regulate a single instance.

A security group is Amazon's version of a firewall that includes some additional features. It allows you to specify certain security settings on an instance specific basis. You have the ability to filter traffic based on IP's (a specific address or a subnet), packet types (TCP, UDP or ICMP), and ports (or a range of ports). You can also grant access to an entire security group. This allows your trusted machines to access each other without having to open ports to the public.