A script conveys the dialogue of the movie, and will briefly describe visuals, although not too much, as this is left up to the director.
There are certain rules as to how to format a movie script. If these are not followed, then it is highly unlikely that it will ever be taken seriously. In order to test the effectiveness of a script, actors will do a first reading of it. If the format is not clear, then it causes the actors confusion, and makes the task very difficult to complete.