What Is Employee Ranking?
Employee ranking is an attempt to evaluate employees relative to each other, by using some criteria to identify the better performers versus the less better performance. The result, at least in theory, is a list of employees in some sort of ranked order.
Often employees are ranked by taking some average or sum of ratings on some set of criteria. Unfortunately, that's not a mathematically sound practice. Neither is it effective in identifying the truly valuable employees from the less valuable. It's also not effective as a method for improving the performance of any individual employee.
For a longer examination of the flaws in how rankings of employees are used see "Why Employee Ranking Systems Lead To Disaster"