If I understand you correctly:
DVD resolution is 720x480 (NTSC; PAL is 720x576 IIRC).
Blu-Ray resolution is 1920x1080.
On top of that, Blu-Ray can handle video bitrates of up to 40 Mbps, while DVD is limited to under 1 Mbps.
On top of that, multi-channel (surround sound) DVD audio must be compressed, whereas Blu-Ray can use uncompressed PCM surround audio, or losslessly-compressed DTS-HD/Dolby TrueHD.
In other words, it looks and sounds much, much better.
However, if you don't have a TV larger than 30", you probably won't notice a difference in picture quality, and if you're using your TV's speakers - or a receiver that doesn't support multichannel PCM or DTS-HD/Dolby TrueHD - you definitely won't notice a difference in sound quality.