Documentary

A documentary is, first and foremost, a non-fiction piece of work, which specifically documents / captures reality. Most documentaries are made because filmmakers do not feel that their subject is being covered adequate, enough or even at all by the mainstream media.
The documentary brings viewers into new worlds and experiences via presentation of factual information on a given subject; the way it is weaved into a compelling, yet truthful, piece of work also defines the documentary.

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Most documentaries have a runtime between 30 and 120 minutes, but there are numerous examples that are both shorter and longer; you could have a documentary that fits as much information into 5 minutes as one that runs for 50 minutes.

Like any piece of filmography, the work should abide by all legal constraints, and should bear in mind ethical restraints as well.
When filming in public places, for example, or on private land, permission should (usually) be gained from the land owner or local governing body; there may be reasons that forbid you from filming in certain areas (sensitive/confidential information, such as in court cases). Permission is also to be gained from those you are out to film, especially if such people are classed as "vulnerable"; this is usually defined by their age or a disability. Some people may be happy for their opinions to be heard/voiced, but wish to remain anonymous, and as such, these should reservations should be understood and adhered to if their contribution is included, as the subject may suffer repercussions if their identity is leaked.
Usually, a release form is signed by and agreed between the production company and the subject, with the conditions being written into this