While HDTV sets are a offer a wider screen than SD sets, they still aren't as wide as some movie aspect ratios. This means you may still see black bars at the top and bottom of your screen (known as letterboxing) when viewing a movie.
When a movie image is wider than the screen it is dispayed on, there are two choices to resolve the issue. One removes part of the image, the other simply shrinks the whole image down.
The way to fill an entire screen with a wider image is to cut off the sides of that image (known as center cutting). This is usually accompanied by artificial pans added to the image in order to keep the action in that center visable area (a procesdure known as "Pan and Scan"). While this does work for many shots, it can create a cramped look or even change the look or "feel" of a scene in others.
If you want the whole image left to right, the only other choice is to shrink the whole frame down until the edges show up on a screen. Because there is nothing above or below, the image appears to have black bars over the top and bottom. These bars are not cropping the image; they're just space fillers.
Some movies don't need to have the sides cropped in order to view the whole image. They were shot using a process known as "Open Matte". This means the original film was shot in a narrower aspect, leaving extra space at the top and bottom. For the theater screens, the image is cropped on the top and bottom, removing the extra space above and below. For the home video market, the cropping is removed. This makes for a reasonable compromise between the desire for some to get the whole image and others to have a "full screen" version on their TV. While the extra top and bottom portions were never meant to be seen, nothing is being removed.
However, if wish to see a movie as it was shown in the theaters, you'll have to put up with some letterboxing, even on a widescreen TV. The good thing is, those bars are a whole lot smaller on a widescreen TV than on a SD screen.