They’re discs that store and read data using pits and lasers. The main difference is that BluRay uses a blue laser instead of a red laser, or bluray uses a higher frequency/shorter wavelength of light than DVD for its laser. This allows a BluRay to hold a lot more data than a DVD. Why a DVD only has a SD resolution movie on it but a BluRay can have an HD to 4K movie on it.
Edit: Added Higher frequency and Shorter wavelength.
Blu-ray is an improved technology (how the disc is made) that allows for a higher resolution, which makes the picture look sharper and more detailed.
DVD is 720 × 480 pixels, BR is 1920 x 1080 pixels.
There is now UHD Blu-ray, which has a resolution of 3840 X 2160 pixels – this is known as “4K”, and is effectively 4 times to resolution of a blu-ray.
8K would notionally come next but the “winning” format is yet to emerge; different companies will try to market their own media until one is widely adopted or has market share.
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