Knowledge is what a person knows, such as employment law or a programming language. Skills are learned, practised abilities, like writing a report or operating a forklift. Abilities are broader capacities, such as reasoning under pressure or spatial awareness. Breaking a role down this way, often through job analysis with people who already do the work, gives a precise list of what to look for and how to test each item.
KSAs turn a general sense of 'a good fit' into criteria that can be assessed and compared. Each one can be matched to a method: knowledge through questions or tests, skills through work samples, abilities through structured scenarios. Defining them before screening begins helps every candidate be judged on the same job-relevant evidence and makes decisions easier to explain later.