Human-computer interaction (HCI) studies how people use interactive systems and how those systems can be designed so intended tasks are achievable, understandable and efficient.

A system can be functionally correct yet difficult to use. Usability asks whether intended users can achieve intended goals with acceptable effectiveness, efficiency and effort in the context where the system is actually used.

Usability is not one universal score. A command-line tool for experts, a public kiosk and a safety-critical medical interface have different users, environments and consequences of error.

Good interaction design therefore starts by identifying the people, tasks and conditions that matter, then evaluates the resulting interface against those realities rather than against the designer's familiarity with it.

HCI treats the interface as part of system behavior: what users can perceive, understand and do determines whether the software's underlying capabilities are practically usable.