A DVD is a silvery disc that contains a movie or other visual media.
This is unknown to most people today as they just grab videos off BitTorrent or PirateBay. Others just hack cable accounts with "123456" passwords. This is all aside from the standard downloads off the internet: cat pictures and porn.
DVDs come in clamshell cases to protect them from dust and scratches unless they have been stolen. Why some people collect these is a mystery, as they have lame artwork that can be stolen separately anyway. And they're full of paper junk like coupons for things that nobody wants. What DVDs are good for is looping them until you and your friends can repeat the dialogue word-for-word, just because you can.
What is really cool is searching for the weird DVD that, if you watch it, you die in seven days? What? It's a videotape? Forget that.