For example if a film is described as being in a 1.85:1 Anamorphic Widescreen format then on a conventional 4:3 (square) format television the player will squash the picture to the correct size and insert black bars at the top and bottom of the picture.
Play the same film on a Widescreen TV and the player will keep the same vertical resolution but stretch it widthways. The image therefore will maintain the correct aspect ratio but also fill the whole screen.
The reason disks are encoded this way is to utilise the maximum resolution of the picture format (PAL, NTSC etc.) without wasting screen lines on displaying black bars.
You will quite often find that the term anamorphic isn't mentioned on the disk box. If you see "16x9 Enhanced for Widescreen TV", this basically means the same thing.
Also you will sometimes see DVD's described as either 16x9, 1.85:1 or 2.35:1. 16x9 means that the disk has been optimised for widescreen TV's while 1.85:1 and 2.35:1 are measurements of width to height ratio. 1.85:1 will roughly fill a widescreen TV, where as 2.35:1 is more of a cinematic format and will cause black bars at the top and bottom even on a widescreen TV.