Ratings are purely subjective. However, there is some logic behind the rating according to the form of art:
Films are rated from a scale from 0 to 10. 10 means the film reviewed is a masterpiece, comparable to the great classics of world cinema. However, the term masterpiece does not limit itself to the classics but also contemporary films are considered for this rating.
Specifically, the rating scale can be explained as follows:
1-2: OK (C-, C)
3-4: Good (C+, B-)
5-6: Very Good (B, B+)
7-8: Excellent (A-, A)
9-10: Masterpiece (A+)
A star rating system is used for classical music reviews, from a scale from 1 to 5 stars. Since the quality of classical music releases is most of the times higher than that of contemporary films, the rating will most likely aggravate between 4-5 stars. This means that usually only quality recordings will be reviewed.
Book reviews do not have a rating system. Any positive/negative aspects will be discussed thoroughly within the review itself.