SD or Standard Definition is the predecessor of HD or High Definition video and is characterized by the smaller 720 x 480 interlaced screen size 4:3 ratio and 480 lines of vertical resolution (720 x 576 PAL). HD video is High Definition Video characterized by a higher resolution image and sound at a screen size of 1920 x 1080 (1080 lines of vertical resolution) or 1280 x 720 (720 lines of vertical resolution) the more lines of vertical resolution the higher the image quality. So from lowest resolution to best is, SD video at 480 lines of vertical resolution, 720 HD video, and finally 1080 HD video. The main considerati on to keep in mind is DVD is SD and Blu-ray is HD so HD video must be down converted for DVD.
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