DVD's only contain standard def video. This became the standard in 1948 to fill a nine-inch TV.
BlueRay takes advantage of 70+ years of technological advancement behind the HDTV standards.
Also: DVD sound is Dolby Digital which is a very good 6:1 compression. BluRay disks carry these sound tracks, but also many titles also carry un-compressed versions of the soundtracks. For most movies, you probably cannot tell the difference, but the un-compressed tracks do sound a bit better on major titles.
The blu ray disc can hold more information on the same size disc as the dvd. With more information on a disc, this brings up the higher resolution of 1080P compare to a dvd that is 480P. Prior to dvd it was the laser disc the size of a vinyl LP and you had to flip the disc to view the other side to complete a movie. This was the first digital source. Hope this will help you out.