Ditto to everything prof said.
professorhat:
However, sometimes a studio may decide to release a film which was in 2:35:1 or 2:40:1 in the cinema as a 16:9 release on DVD or Blu-Ray (mostly these studios are evil in my opinion) by chopping off the sides and you'll therefore get a fullscreen image. The downside is you're not getting the full image the director intended you to see and, in some cases (though admittedly very rare cases), this can actually affect the story as critical information can be missing.
It's worth pointing out, though, that this process (known as 'panning and scanning') is very rare practice nowadays, as far as the major studios are concerned at least.
In the days of 4:3 TV screens, putting a film on DVD in its original 'scope' aspect ratio could often leave almost two-thirds of the screen blank - so studios produced separate 'fullscreen' (4:3) and 'widescreen' (16:9) editions of many titles. But with 16:9 screens now being the norm, studios have the sense to present films in their original aspect ratio, albeit still leaving (far smaller) black bars at the top and bottom of the picture.
As another aside, I'd remind all that the next
big Blu-ray release (that would be
Avatar) was shot in 1.78:1, meaning that it will be the perfect fit for all widescreen displays
.