Basically, blu-ray is denser than DVD. It needs a special blue light to read, hence blu-ray.

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.

To add onto what others have said, blu rays have more DRM, so the movie industry loves them. Playing a bluray on a PC can be a pain in the ass

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.

TBF originally it was also a pain in the butt to play a DVD on a PC.

But at the time in DVD life cycle we atleast had an easy time playing them on PC, the fact that software to play back legally owned blurays need to pay a subscription to license the codec is nuts.

I wouldn't know, I incompletely skipped Bluray. (I was gifted a single disc, but have nothing to play it on)

I'm old fashioned. I like to format shift my stuff, which is illegal with blurays. Technically legal for DVD/VHS.

Blu-ray uses a shorter wavelength laser than DVD, not larger. The Blu-ray wavelength is 405 nm compared to the 650 nm wavelength that DVD uses.

I'd like to add the clarification that color is how our brain interprets light's wavelength. So 'blue' and 'short wavelength' mean the same thing, when talking about light.

The laser in a DVD or Blu-ray drive is used as a 'needle' to detect small pits on the disk, which encode 1s and 0s. A red laser has a long wavelength and is akin to a fairly blunt needle. It won't detect very small pits, so the pits in the disk need to be fairly large. Which limits the storage capacity of the disk.

With a blue laser, which is a much finer 'needle', the pits can be a lot smaller, so a lot more data can be stored on the disk.

There are also other factors like how fast the disk spins, and how close the pits can be to eachother. The accuracy of electronics manufacturing has improved since the advent of dvd's, so we can now make smaller devices with fewer chances of important errors. How efficiently the data on the disk can be compressed has also evolved substantially since the advent of DVD, further increasing the amount of video a disk can hold.

Ah, late night minor error. Higher Frequency, Shorter Wavelength.