There are three kinds of trailers you should be familiar with: teaser, theatrical trailer and sales trailer. A teaser is approx. a 30 second spot that "teases" the audience with your story. Teasers may be released even while the film is being edited to whet the appetite of the audience.
Here is a teaser trailer for The Dark Knight:
A theatrical trailer is about 2 to 2 1/2 mins and it's typically the trailer used to hook audiences. Theatrical trailers show before films at the movie theater or on TV or the Internet. Thus they need to be for all audiences (no nudity or swearing or extreme violence, etc.).
Here are two theatrical trailers for The Dark Knight:
Sales trailers, on the other hand, are usually for buyers of the film. They tend to be longer (almost 3 mins or more) and contain the goodies from your film that you wouldn't find in the theatrical trailer, i.e. nudity, blood, scenes of high production value that give away too much of the story, etc. Buyers want to see what your film has to offer in one short clip.
I don't have a version of the sales trailer to show you from The Dark Knight. That trailer would be used by sales agents at film markets like the ones at Berlin or Cannes or Los Angeles (American Film Market or AFM). Just imagine a longer clip with more of the scenes that really give away all of the major selling points, like the incredible VFX, etc.
Many independent films have a hard time affording an experienced trailer editor. I understand that predicament but I would suggest that you do your best to try to find someone with trailer cutting experience.
A trailer editor will have been trained in how to "sell" a film. That knowledge and experience can directly result in sales of your movie. I have heard countless stories of distributors buying movies based on solely seeing the trailer. Now that's an effective sales tool.