Blu-ray discs typically cost quite a bit more than ordinary DVDs. For example 'The Lion King' on Amazon is priced at £6.98 for the DVD but at £12.99 for the Blu-ray disc.
To play a Blu-ray disc you, unsurprisingly, need a Blu-ray player. The advantage of doing so is that, as long as you've also got a TV set capable of displaying HD ('High Definition') images, you'll get a much better picture quality.
You can use a Blu-ray player to play ordinary DVDs (although, of course, you won't then get the better picture quality) but you can't use an ordinary DVD player to play Blu-ray discs.