Difference between large and original images

Hello.
Can you please tell more about images and size:
1) thumbnail
2) large
3) original

What the difference between large and original?

I disabled option before migration from JS to ES:
- Allow Viewing Of Original Photo
- Allow Downloading Photos
But every new photo has large and original version.

What if I don't allow users "download" images and don't need to keep original images? How to remove it?

Thanks.