Video signals are generated using horizontal lines. An interlaced picture draws every other line and alternates between drawing odd lines and even lines. A progressive scan picture draws every line in sequence. Therefore, a progressive scan video signal sends twice as much data than an interlaced signal each time it draws an image on the screen.
Before DVDs and HDTV became popular, interlaced video was the norm for television. Standard definition broadcasts were interlaced, since it was a more efficient way to send video data. Since the human eye has a hard time detecting video interlacing, an interlaced signal that refreshes at 60 Hz (times per second) is easier on the eyes and produces less flicker than a progressive scan signal that refreshes at 30 Hz.
Still, if a progressive scan and interlaced image are both projected at 60 Hz, the progressive scan image will usually appear slightly smoother. Video that contains fast motion makes this difference more noticeable. For this reason, the DVD and HDTV standards were developed to support progressive scan video signals.
When you see video formats described as 480p or 720p, the number indicates how many horizontal lines of resolution the video signal uses, while the "p" indicates it is a progressive scan signal. Similarly, the 1080i format contains 1080 lines of resolution, but is interlaced. Both 720p and 1080i are used by HDTV.