360 Feedback: The BEST Performance Review System Top Companies Use
The goal of 360 feedback is to generate as holistic and fair a picture of employee performance as possible.
It does this by getting feedback from all directions:
Above (your manager)
Below (your reports if you are a manager)
Laterally (your teammates)
This system prevents employee performance from reducing down to sucking up to the manager, which is the case in more traditional companies that aren't using this system.
360 feedback also makes the power dynamic between manager and report more equal. A manager will have a bad feedback packet if their reports all share bad feedback about them. This holds bad managers accountable.