Trailer terminology and Definition

Definition
A trailer usually consists of a sequence of carefully selected scenes from an upcoming blockbuster or "indie" film to promote it to audiences. Trailers can appear in many forms from teaser trailers to extended trailers and usually encapsulate the most noteworthy and entertaining parts of the film without giving away any spoilers of the plot or in sequence order. To achieve this trailers are under strict guidelines of the duration being under two and half minutes, which is the maximum length allowed by theatres. This maximation is relaxed once a year where a studio may extend a trailer or exceed this time if they feel that it is necessary to successfully promote the film to the desired audiences/demographics.

Plot Summary
Most trailers have a three-act structure; similar to the structure of a film with a beginning that lays out the premise of the story, continuing to a middle act 2 which drives the story further that mostly ends with a dramatic climax. The final act 3 usually features a strong piece of "signiture music" (recognizable song or powerful piece that fits the film and trailer edits and sequence). This last act often displays a visual montage to accompany the music of powerful moments of the film, sometimes featuring a cast run to help sell the movie, especially if they are noteworthy stars.

Voice Over 
A voice-over is a narration used to briefly set up the premise of the movie and provide explanation when necessary. It is usually used to enhance the audience's understanding of the plot in a short space of time.

Music 
Music is usually an important part in setting the feel and mood portrayed to the audience and is not usually from the film itself as this music may not have been composed at the time the trailer is released. The music piece in the trailer may be music from other movies, popular or well known music (recognisability- usually chosen for link to an appropriate lyric for example), "library" music (composed by an independent composer), specifically composed music or songs that are similar to originals but may be too expensive to license.

Cast, Crew and studio information
A cast run is a list of the stars that appear in the movie. This can also be the same for the director or producer if they are well known. Also most trailers include a billing block at the end. This a list of the cast and crew which appears on posters and print publicity materials and also usually on-screen at the beginning or end of the movie. As well as the billing block and cast run, studio production logos are featured in trailers (usually placed at the beginning, since the 1970s). There are often logos for the production company and the distributor of  the film.

Technical Elements
Video resolution: Trailers are usually presented in the same way as the feature film, whether it is in 35mm film or a digital format. Improvements in internet connection means that many trailers can be distributed at resolutions up to 1080p HD quality.
Sound Mix: most movie trailers are presented in Dolby Digital or another multichannel sound mix. Stereophonic sound enhances any sound effects or music, which makes it one of the focus points of modern trailers.