Genre refers to categories of artistic works such as literature, music, and film. Genres are defined by conventions that change over time as new genres emerge and old ones fade away. Films can be categorized by their themes, settings, moods or formats. However, defining film genres is complex as most Hollywood films blend elements of multiple genres. The need to categorize films into genres is important for marketing films to audiences and enabling discussion, but genres are not pure and film identities can be ambiguous.