One way to get a widescreen picture is to use a normal 35mm frame, approx. 1.35:1, and add black bars to the top and bottom to get the correct ratio desired. This matte process is used commonly with 1.85:1 or 1.66:1. When creating a home video from a movie that was originally matted there are two choices, Open Matte, Pan & Scan Or a combination of both.
There are two ways to matte a picture, either a hard or soft matte. A soft matte is filmed with the films ratio 1.35, then black bars are added after to the film copy or the video transfer. A hard matte is shot with the bars in place and there is no additional information underneath the aspect ratio chosen. You will commonly find that hard mattes are used for CGI effects, because the studio doesn't wish to pay for more area of the image to be rendered than is shown to the audience. The result is a video release that has an Open Matte during non-effect shots but is Pan/Scanned during effect shots because there is no additional information that contains the special effect.
A movie originally filmed in a soft matte can be transferred to video with an open matte. The video release would have the matte removed (the black bars) exposing new information that wouldn't have been seen in theaters. This can lead to distraction since there are certain things the director may not have intended you to see (like microphones, etc). This is by far a better way to view a video than Pan & Scan, which actually takes more information away, but not desirable.
This open matte is for the most part just open on top and bottom, but notice that there is a little bit of information lost off the sides of the image.
Open Matte + Pan & Scan
This open matte for the most part has a little additional information added to the bottom but there is a bit of information that has been cropped from the sides too. That's why this is more of an Open Matte + Pan & Scan. Not only are you seeing stuff the director may not have intended you to see, you are missing out on some stuff that he wanted you to see!