You have to understand aspect ratios and do some research on them.
Typical aspect ratios:
4:3 (1.33) - This is the format of old television. For years and years, this was the standard for TV. Some very old movies were shot in 4:3 aspect ratio.
16:9 (1.78) - This is the format of HDTV. Some people call it 'widescreen', but that's like the word 'hot' - it doesn't mean anything. It is the format that is (more or less) exclusively used by broadcast high definition stations. It is also very close to one of the movie standard sizes. Movies generally are not shot in 1.78 aspect ratio, but all current LCD and plasma displays worth discussing are 16:9 aspect ratio. So, this is typically the format for home theater projectors.
1.85 - This is the second most common format of movies. It is very close to 1.78 (HDTV) aspect ratio, so it fills up a projection screen or TV nearly completely.
2.35 - This is the most common format of modern movies. Think Lord of the Rings, etc. It is wider than 1.78 (HDTV) so it has black bars above and below the movie when showing on a projection screen, but this is still the format of choice for directors who want that big impact to really fill your vision. Sky is boring, ground is boring, but wide shots of the scenery is great to see.
I did this write up a while ago, but it somewhat explains things: http://www.avintegrated.com/aspect_ratios.html
I really need to update that with Blu-ray Disc! FYI: All Blu-ray Discs at this time are 1.78 (HDTV) aspect ratio, but the movie on the disc is quite often different. It may be a 2.35 movie put onto the 1.78 aspect ratio disc, so black bars are encoded above and below the movie.