An image shot using IMAX film contains much more resolution than a normal 35mm film. However, because IMAX cameras are heavier, noisier, and more expensive, no feature film has ever been photographed in IMAX.
Normally, when feature films are shown in IMAX, they are "blown up" from the regular 35mm negative. They recieve some benefits of the format (bright, steady images), but they do not have the same sharpness and detail because they are limited by the resolution of the original photography.
Even worse, modern films (IMAX blow-ups as well as regular 35mm releases) are often limited by the resolution of a "digital intermediate," where the film undergoes post-production work (FX, color balancing, etc.) in the digital realm. Because digital intermediates require putting the entire film onto a hard drive (an expensive and time-consuming process), they are often done at lower resolutions than the original photography.
Enter Director Christopher Nolan and Cinematographer Wally Pfister.
They shot 28 minutes of The Dark Knight using IMAX cameras.
And they did not use a digital intermediate for the footage they shot in 35mm, so all of the detail that was recorded was preserved during post production.
What does this mean to the viewer?
If you see The Dark Knight in an IMAX theatre (recommended), 28 minutes of the film will be sharper and more detailed than any feature film in more than 15 years. The IMAX sequences will fill the square (ish) screen and the rest of the film will appear "letterboxed." And thanks to the extra efforts taken by Nolan and Pfister, those 35mm sequences will look as good as they possibly could.
If you see The Dark Knight in a quality 35mm theatre, the image will be sharper and more detailed than most films. The scenes shot in IMAX will be cropped at the top and the bottom, because IMAX is a square (ish) format, but they will be even more detailed than the rest of the film.
If you see The Dark Knight on video, either at a "digital projection" cinema or on your television at home, you are really missing out.
List of theatres showing The Dark Knight in IMAX
UPDATE: Minor edits have been made to this post after reading this excellent article: Batman Looms Larger. Please check it out if you want more specific details, but if here's a paragraph that really captures the essence:
“It’s ironic,” muses the cinematographer, “because many filmmakers are trying out digital cameras that actually capture less resolution and information, and we’re going in the opposite direction, upping the ante by capturing images with unparalleled resolution and clarity.”
Bravo to Christopher Nolan and Wally Pfister for keeping the art of FILMmaking alive, and bravo to Warner Brother for supporting them.