An aspect ratio wider than 1:33:1 (4:3). Some people think that regular TV is "square," when it is actually 4:3. Most movies are shown in either 1.78:1 (16:9), 1.85:1, or 2.35:1. TV shows are usually shown in 1:33:1, but some are shown in 1.78:1, such as The Sopranos and ER. If the aspect ratio of the content you are watching is wider than your TV, the picture is letterboxed. Many people don't like widescreen because they think that you don't see the entire picture, which is not true. If they had a 16:9 TV or were watching it in a movie theater, they would not think so. Those people will buy the "full screen" pan & scan versions of DVDs, which actually cut off 40% of the picture.
Widescreen DVDs are better than pan & scan because you see the whole picture.