All user reviews on the site can be rated. At the bottom of each review, a prompt asks "Was the above review useful to you?" followed by Yes/No buttons, and registered IMDb users can cast their vote.
Please note that "useful" doesn't necessarily imply "positive" or "favorable". A review can be useful even if it rips the film to shreds because it can help readers decide not to waste their time or money watching it.
Likewise, to find a review useful doesn't mean that you must agree with it. A well thought analysis of a film can still be useful and shape your opinion differently or bring a novel point of view to light, even if you ultimately disagree with the author's conclusions.
The aim of the 'useful' feature is to help valid, well written reviews surface at the expense of less interesting ones. We think that our users are ultimately in the best position to decide what's good or not for them and that their opinion should be given consideration when displaying user reviews on the site. Therefore if a majority of users gave a positive opinion of a review with which you disagree, it really does mean that that review is more useful than others, even if you are not among them.
Reviews that are rated as being the most useful are more likely to be featured on the film's title page.