The direct video disk or DVD as it is more commonly known was invented to replace video tape. The advantages of the DVD disk over tape was:
1, Better picture quality
2, It is digital so the disk could have a DVD menu and therefore the viewer is able to skip to any part of the film they want to watch, almost instantly. With tape you would have to wind the tape forwards and backwards until you found it.
3, The more you watch tape, the more the quality dropped as the tape ribbon became gradually eroded.
After DVD came the blue ray, which is essentially just a high definition version of DVD. DVD is composed of frames which are 720 x 576 pixels, where as blue ray is 1920 x 1080 pixels, which is almost four times the number of pixels. These pixels obviously give a much better picture quality.
Blue ray is called blue ray because the disk is multi layered in order to fit all the necessary high definition information on the disk. The laser beam that can scan multiple layers of the disk is blue in the spectrum of light and therefore the disk is called blue ray.
Now films are beginning to be produced on 4k disks, which have essentially four times the pixels of blue ray, making an even better picture quality.
Obviously these higher pixel disks are more expensive than standard DVDs, but when deciding which to buy, one must think about where you are going to view it. If you have a 20 inch TV, then a standard DVD will be fine because the 720 x 576 pixels will still look great on a screen of that size. Once you move up to a 50 inch TV, then the pixels will start to become stretched and the image will look a little fuzzy, so it is worth thinking about paying the extra money for a blue ray disk.
There is no point in watching a 4k disk on high definition TV, because you will not get the added benefit. You should only buy 4k versions of films if you have a 4k TV, because only a 4k TV can display all the pixels
In summary, the world of media moves fast, as televisions up grade so to do the disks. One must remember that the media should match the television that you are watching it on, or else you may end up paying for technology that does you no good.