How does a Blu ray DVD picture look better than a regular DVD movie?

I mean how can it get better?
I understand that a DVD looks much clearer than a VHS Tape, but how much better can u do to make it clearer than a regular DVD?

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1. can an hd/blu ray dvd play on a regular dvd player and still have the same picture quality?
2. can an hd/blu ray dvd play on a regular dvd player and still have the same picture quality?
3. Do movies on a blu ray dvd player look the same as a movie in HD?
4. Do Blu-Ray DVD’s have better picture than a film at a movie theatre?
5. Which Blu ray DVD/movie has the absolute best picture and sound?