Films come in a wide range of topics and moods. They are pushed into certain genres. Sometimes movie-goers react to a film in ways that are not expected, perhaps seeing a serious film as a comedy or not finding a comedy funny. How does one know what type of a genre a film should be? Typically this is determined by the story that the film has and the way it is told.
Drama films usually work on deep emotional exploration of characters that are realistic, placing the story into a believable location emotionally so that lessons about various social issues and emotional themes can be dealt with. These commonly include drug and alcohol abuse, racial or religious tensions, economic hardship, cultural/social/class friction, abuse, or corruption. Characters are often placed in situations where they must overcome some hardship within their own makeup, the culture around them, or some natural event.
The serious fashion that these varied themes are addressed often can pass off somewhat preachy concepts about how certain things impact people, society, culture, or nature as entertainment. Many times these themes can address sensitive issues long before society is willing to speak about them openly for fear of being marginalized for controversial stances on certain topics. Dramas allow for the exploration of the human condition under all types of situations, showing us how people can be heroic, dastardly, charming, and downtrodden, often all side by side. No other type of genre allows the full exploration of human behavior.
Because Drama is a very broad genre there are many many sub-genres in existence. This covers romantic, sport, historical, law/courtroom, and criminal drama types, among others. A well done drama will focus one one or more characters at a crosspoint in their lives, exploring the situations, emotions, and choices that they make in dealing with a specific crisis point. Dramas commonly have a sense of tragedy about them, touching on or digging into painful topics. This breadth of emotion often allows actors to develop fully into their characters in ways that others genres do not allow for.
Drama films have been nominated for Best Picture and other awards more often than any other type of genre. Not only do the serious topics hold sway in this fashion, but usually actors give their best performances in such a setting. Drama films are actually made more frequently then any other type of film because of the many sub-genre that exist.