Unlike regular trailers, they are very short in length, usually 30 to 90 seconds, hence why they are called 'teasers'. They also are likely to be shown long before the product it is advertising is released in order to raise awareness.
Here are some examples of Teaser Trailers
The Texas Chainsaw Massacre
This is a short trailer for the 1974 B Movie the Texas Chainsaw Massacre. Unlike the other trailers I have studied here, this film does not use the convention of on screen text, with the exception of the title. It instead uses a voice over which uses phrases such as "the most horrifying motion picture I have ever seen". It shows shots of dead bodies with a camera like noise, giving it an almost documentary style feel. The shots shown day feature shots of a terrified woman screaming and being thrown to the ground, with loud noises in between. There is also a horror film cliche of the full moon. We also see action shots, such the woman jumping out of the window and being handled by a large man. towards the end of the trailer we see a low angled shot of the large man weilding a chainsaw, making us see from the victim's perspective.

This is a graph from IMDB.com of the average votes from different age groups on a scale of 1 to 10. The highest approval rating came from females under 18. However, we can see that less females in total have watched the film, but the younger viewers enjoy it more. This makes sense, as the protagonists seen in the trailer appear to be young as well, so it's possible that the film was designed for a younger audience.
28 days later
http://www.youtube.com/watch?v=fb1OXb0pFFQ&feature=related
This trailer is 1 minute 30 seconds long. It shows text seperating different clips from the film. The font for the text used looks damaged and we can hear screaming and a siren. This suggests violence or war, some sort of epidemic. We then see a fast POV shot with green and red lighting, which suggests infection and danger. Thunderclaps can be heard between shots, which is common in horror films. Another cut shows a scene with flickering lights showing the shilouettes of fast moving figures in the background, which will surprise us and cause panic. White text above a black background describes the plot details, with words such as "INFECTION", "Day 8, EPIDEMIC", "DAY 15, EVACUATION", the latter of which has a police siren. We then see the films title, "28 Days Later" and a change to bright colours as we see a man arise from a hospital bed. Unlike the previous scenes shown, this has bright lighting, suggesting that he is the hero. He then walks into a completely empty London, suggesting a post apocolyptic world. we then see the main title, with red demonic eyes followed by a "Days are numbered" slogan. Finally we see the word "SOON" with a quick scream sound, shocking us.
REC 2
This is a very minimilistic trailer. It contains only one POV shot from a man walking up the stairs, looking at the blood, with another man infront of him. We can tell that they are most likely to be a team investigating the area. We then hear a scream and the screen shakes and goes black. We can tell that this is a suspense horror film
The Dark Knight
This trailer is different from most teasers because it shows no footage from the film at all, just quotes from the characters. Instead, we see an animation of the Batman logo crumbling, until a joker card is shot at the audience. This is very simplistic and would appeal to the pre existing fanbase of the film because they know what to expect. This is not useful for my film because I do not have a pre existing audience.
Drag Me To Hell
Drag me to Hell was Sam Raimi's return to the horror genre after his SpiderMan films. This is a more traditional trailer than the other 3 here as it shows different clips from the film, rather than one scene. It begins with a loud noise with the film companies logos, followed by eerie percussive music. The music also features a high pitched shreak like noises, which disturbs us. The first thing we see is a shot of a seance, complete with chanting from the participants and dark lighting. We see fast moving shadows in the background and a shot of a shandelere shaking. There is an attractive blonde girl shown in distress who is likely to be the main character. We see spiritualistic imagery, such as a crusifix, creatures and demon sillouetes. In between these clips we are shown favourable film reviews in black text on a dirty white surface. Towards the end of the trailer, the music and shots speed up until we here the quote "burn in hell for eternity", followed by the girl becoming engulfed in flame, screaming. We are giving a final shot of a screaming woman, shocking the audience. A narrator then announces the films title and release dates. This trailer has a good structure and pace for a horror trailer. Although it uses computer effects which I am not capable of replicating, I believe that this is an excellent example of a horror trailer.

This film, like the Texas Chainsaw Massacre, appealed mainly to people in the younger agegroup.