This is what you read before you decide what movie to go see on the weekend. They are printed in every major newspaper that is worth its salt, and are also available online, in magazines, on TV, and everywhere that there is media (as I'm sure there are more on the way). Like any industry, criticism is an important component in the film industry.
These are written by film critics. While careful study of film, the making of movies, and film history are important for a complete understanding of what a critic is talking about, it is also possible to write movie reviews even if you are a regular joe. Think about it: while you would like to educate your audience, you would also like to entertain them. I make it my goal when writing a movie review to make it interesting and compelling to read.
I use a formula when writing reviews. It is simple. Some people like to only skim the beginning of a review to only get a sense of its quality without finding out the actual plot. For those people, consistency is good.
A good formula, then, is this: start by giving a sense of your rating of the movie. This can be written last. Tie your words in with the film. You want people to read the first few paragraphs of your review and get the feeling of the movie -- people decide whether or not to see a movie based on feel as well as quality.
Then, summarize the plot. Don't go too far into it. If you give away anything important, you will be shot. Movie reviews are not the place for serious criticism and discussion of the movie, which would include spoilerage.
Then, answer any or all of these questions: What did you think of the director? What did you think of the lead actor and actress? Was anyone's performance particularly inspired? This is the type of thing to bury lower in the review, because lots of people don't care.
Lastly, wrap it up. I like to answer the question "Why should I see this movie?" There are always people who will like any movie, you just have to figure out who those people are, and put it down.
I wish I were able to follow this formula, but I'm not always that great at following instructions. Anyway, the most important attribute of my reviews are their entertainment value, and the fact that people know I stand by my opinion, as far as I can tell. I just like to make people laugh.