AWS OpsWorks, formerly Scalarium, is a chef-based system that deeply integrates with EC2 (security policies, key pairs, etc). OpsWorks acts as web console to configure your entire system, and as a command hub for running chef-solo on all the instances.
A stack represents a collection of inter-connected and inter-configured services.
A stack has lifecycle events which are triggered by any change to the stack configuration (such as, adding a new instance to a layer, or changing the hosts of an application). In this way, the stack acts as an event hub, notifying every instance of any changes.