The movie reviews also include the Motion Picture Association of America (MPAA) rating. You might know that the movie rating is determined by a MPAA Review Board, and that the producers of the movie are not required to submit the movie to be rated -- it is voluntary.
But the ratings are very useful to many people, from kids to parents to video sellers and rental stores. So most movies are submitted for rating. These are the ratings you are probably used to seeing: G, PG, PG-13, etc.
For more information about the MPAA ratings and what they mean, you can go to the MPAA's Movie Ratings, where you can also search for the rating on any movie you are interested in.