Film Trailers? - What are they?

When you google 'what is a film trailer' the results are all roughly the same with the words switched up. The top result from Wikipedia is:  trailer or preview is an advertisement or a commercial for a feature film that will be exhibited in the future at a cinema.' Trailers are made to give the potential viewers a sneak preview of the film to give an idea to the audience about the characters and the storyline without giving away the whole plot. You can also have teaser trailers which are shorter-length trailers which only portray one part of the film! Trailers are made to advertise the film across many different media platforms, trailers can be played in cinemas before other films or they can be played on tv and now with new technology they can be played on electronic billboards, phones and tablets!