A Blu Ray works for Blu Ray disks, standard DVDs, and even CDs. So, you can pretty much play anything on them. The Blu Ray format is the new standard for HD programming on a disk. So, it may eventually be the only type of disk available for future releases of movies (like how they don't make VHS versions of all programming any more). And, the only way to watch it would be with a Blu Ray player.
Also, the Blu Ray will play full 1080p Blu Ray disks, and also play 720p HD DVDs, upconverting them to 1080i (if you use an HDMI cable). The HDMI cable also passes digital audio for a true surround sound for the Blu Ray disks.
The disadvantage is that a Blu Ray player that plays standard DVDs may have problems like video artifacts (ghost imaging and video tearing) or refusing to play a particular DVD altogether. And, you usually have to spend at least about $300 for a Blu Ray player that can handle DVDs without these issues.
If you want to see Blu Ray disks being played on HDTVs, go to some place like Costco, where they have a bunch of them on display. They will have all the same programming on them, which is usually a full 1080p Blu Ray disk.