Any film that is short is one that will not be considered as a feature film. There is no definite line drawn to decide this length, but in general it is accepted that a film that runs less than forty minutes, including the credits, can be considered as being one of the short movies.
Most short films are shown as part of the presentation of a feature film, though some film festivals do screen them independently. Most short movies are made on very small budgets, may have no commercial angle in mind, and may get funding from non-profit organizations or depend on personal funds. It is a genre that is used by many movie makers to gain experience, as well as showcase their talent, without having to break the bank.
Short movies require having a complete story, and this means that the film must have an introduction, a body and then the conclusion. It can help if the story line is simple and one that does not require too much of elaboration for it to be understood. It will help if the number of characters are limited, because bringing any character into a movie, does require some time to explain his or her relevance to the plot. The best short films are those that have a single idea and allow the movie to revolve around it.
A short movie becomes more effective if it has some sort of suspense built into it, and not everything is explained to the audience in the beginning. This allows interest to be retained, and if the subject matter of the film, does create questions in the mind of the audience, their attention will be drawn to the movie, in the anticipation that the questions will be answered. We have already said that fewer characters help short movies, as this can also help to reduce the number of actors, and hence probably the costs. A story must be completed or at least brought to some sort of conclusion, which will not allow the audience to feel cheated or unsatisfied. You can create a story with an open ending, but if your characters are properly fleshed out, the audience will imagine its own ending.
Many cartoon and comedy films have always been short movies that complete a particular story within the short time that they allow to it. The short movie format is seen to be very effective in small advertisements, and many people in the advertisement industry, find that their messages are more effective, if their short film has a story line, where the product or service being sold is just incidental. This same technique is also used by video makers for creating songs and music. The internet has been largely responsible for the great popularity of short videos, as this is a format that people do find the time to watch.
Short movies are rapidly being considered a category by themselves and there are festivals that do encourage only such films being shown, some of which may not even last for three minutes.