The Rating add-on allows your users to rate and review channel entries. Along with the obvious, Rating also allows you to display ranked lists of entries, show detailed stats for entries, allow your users to edit their reviews, vote reviews up/down, manual and automated moderation, and more! You can also accept both member and Guest ratings.
Rating also contains it's own set of custom fields, notification templates, and member group/channel privileges. Having your own custom rating fields allows you the flexibility of having as many rating and review fields as you need.
The Stats function is extensive, and allows you to output full stats on each entry as well as individual numeric rating fields. Numeric ratings can be counted, totaled and averaged.
Additionally, you can also use Bayesian (weighted) method of ranking your entries. It makes some calculations on your site and weights the entries a little differently. For example, an entry with an average rating of 4.6/5 from 30 rating submissions would likely be ranked higher than that of an entry with average rating of 5/5 from 3 rating submissions.
NOTE: Documentation for Rating 2.x can be found here.