Years ago, the FAA developed the Practical Test Standards (PTS) to provide skill performance metrics for each Area of Operation and Task.
The PTS is still a sound approach. Over time, though, the PTS has acquired a number of “barnacles” – things like overlapping or redundant tasks, and a long list of undefined “special emphasis” items.
The ACS is basically a souped-up version of the PTS. It adds task-specific knowledge and risk management elements to each PTS Area of Operation and Task.
The result is a comprehensive presentation that integrates the standards for what an applicant needs to know, consider, and do in order to pass both the knowledge test and the practical test for a certificate or rating.