Categories are a way of objectifying an inherently subjective matter. Everyone feels differently about a particular movie. Some would rate a movie high in action, others might think of it more as a comedy. This is what the categorization system attempts to answer.
Each user may set their own category via the Movie View Form or the Add Movie Form. There are 19 categories with a slider for each one. There are 11 possible values, which are also color-coded:
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
with 0 meaning 'not at all' and 10 meaning 'completely'.
Set each slider to the magnitude you feel describes the movie in question. This will be stored and can be used later to find similar movies or find movies in a particular category.
When you first categorize a movie, your categorization will be set to be public or private based on your default setting. Public categories can be viewed by any other user (if they have privelege to view that rating level). Private categories can only be viewed by you, with one slight exception covered in the Movie Search Form.