Performance appraisal is the process of systematically evaluating an employee's performance and providing feedback. 360-degree appraisal involves gathering feedback about an employee from subordinates, peers, supervisors, and sometimes external stakeholders. It provides a more balanced and objective view of performance from different perspectives. While it can be an effective development tool, 360-degree appraisal also has some disadvantages like being time-consuming, costly, and potentially damaging to employees' self-esteem if the feedback is too critical.