There are many sources for movie reviews online, including RogerEbert.com, Variety, Rolling Stone, HitFix and MovieWeb. Sites like Rotten Tomatoes and the Movie Review Query Engine provide links to many of these online reviews.
Both Rotten Tomatoes and MRQE give cumulative scores to movies based on a variety of professional critic reviews online. MRQE, which is the largest searchable database of online movie reviews, assigns a number between zero and 100 based on the average favorability of reviews. The number is associated with a grade from A to F, with an A given for extremely favorable reviews.
A Rotten Tomatoes cumulative score is given as a Tomatometer rating. A Fresh rating is given for better movies based on the published reviews of approved professional critics. Rotten Tomatoes also gives cumulative ratings for TV shows.