Rating is a new feature introduced in SharePoint 2010 that can be added to any SharePoint List/Library. It helps users to evaluate and authors to get feedback. In SharePoint 2007, we have write code and deploy it as feature for rating.
How to enable Rating in SharePoint 2010?
It's very straight forward to enable rating in SharePoint 2010 lists.
- First let's create a Custom list named "Search Engines". If you already have list/library then you can skip this.

- Once created, open the list select "List Tools" Tab -> List Tab -> List Settings.
- Under General Settings select "Rating Settings".

- Under Rating Settings section select "Yes" for "Allow items in this list to be rated?"

- Now a new field "Rating (0-5)" is added to the list. But the actual field name is "AverageRating".
- Add some items in the list.

- When move mouse over the Rating field it'll show message to add rating. You can select the rating as required.
The same way, rating can be enabled on document libraries. Once rating given to an item then it'll be stored in database. Every user can provide their own rating and average will be displayed when viewing the list. There is a specific service("User Profile Service Application – Social Rating Synchronization Job") running behind to calculate the average of ratings.
This job executes every 1 hour by default. If required can be executed manualy by "Central Administration -> Monitoring -> Timer Jobs -> Review Job Definitions -> User Profile Service Application – Social Rating Synchronization Job -> Run Now" or change the "Recurring Schedule" as required.
Once the Synchronization job executed then users can see the average ratings (Blue Stars).
Before Synchronization Job execution.
After Synchronization Job execution.
Move mouse over blue star ratings to know your Rating (only if you have already given rating).
Note: If a user giving rating which he already given, then his previous rating will be discarded and new selection only getting considered.