HD DVDs
 

If I were to buy HD dvds would I be able to play them in my normal DVD player, I know they wouldn't be of high quality, but I was just curious, so that I would be able to take my DVDs to friends houses and watch them on their normal DVD players.

no.

The FAQ has more info.

There's some DVDs that are HD DVD on one side and regular DVD on the other. Those, you can watch the DVD side of.

But the short answer is "no".