How AWS Config Records Resource State

AWS Config is a service that records the configuration of your AWS resources over time and lets you assess whether they comply with desired settings. Where CloudTrail records the API calls that change things, Config records the resulting state of the resources themselves. The two are complementary.

The core unit is the configuration item (CI): a point-in-time snapshot of a resource's attributes, relationships, and metadata. Every time a resource changes, Config records a new CI. This stream of CIs forms a complete history of what each resource looked like and when.