Documentary

Documentaries are films that follow the occurrence of an event, and report on it in a non-fictional manner and are generally aimed to inform. They often contain interviews with experts and are based around certain areas of particular interest such as historical events, topical news, obscure trends, and anything that people might like to be informed about.

Generic conventions:

• Documentaries often include reconstructions or re-enactments, people re-enacting incidents that have already occurred but perhaps have not been recorded previously, for example, if someone wanted to create a documentary on crime or more specifically something like muggings, they may re-enact a mugging to demonstrate it. 
• Voice-overs are a common feature within documentaries, for example David Attenborough's nature documentaries, he commonly does voice-overs while the camera focuses on creatures or aspects of nature.
• Interviews are often featured within documentaries as well, usually with recognised experts in the field that the documentary is based on.
• Breaking of the fourth wall is often prevalent within documentaries, with people often addressing camera crew, or the audience directly, explaining the current situation to the viewers, possibly commenting on events that are unfolding in the scene.

Generally documentaries aren't really suited to short films as its difficult to cover any topic in any great detail in a short space of time, trying to explain something like the skater subculture for example just wouldn't be possible.
However if someone wanted to give a bias view of something, for example portraying skater subculture as negative, it may be a good way to influence what people think of it, by showing only one side of what it is and what takes place within it, they could cause people to look upon it poorly and if this is the creators intention it may be used to great effect.

If I was to do a documentary it would probably be on the skater subculture, purely because it's a widely misrepresented and in my opinion 'hard done by' group of people of which 99% mean absolutely no harm to anyone, and I would like a chance to represent these people in a way that truly reflects who they are and what they are.