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A bit more about interlaced vs. progressive:
Interlaced was used for older technology. Screens and monitors worked by constantly refreshing the screen at a very fast rate. Because of the way that the human eye works we would not notice the fact that the screen was not refreshing in a uniform manner, however if you ever aimed a video camera (or even a regular camera and took a picture) at a television then you would notice that it looked strange, because the camera's own internal refresh rate was faster than that of the screen.
Interlacing video took advantage of the screen refresh rate. Video is simply many still frames, all played very quickly. With interlacing, one of those frames had every other row of pixels deleted. The next frame would also have every other row of pixels deleted, but it would offset that of the previous frame. The third frame would be the same (every other row of pixels deleted, but offset from the frame before it), and so on for the entire video. The benefit of this was that a lot of data could be cut out, which was important for broadcasters (both over the air and through cable). Given how fast each frame goes by, combined with the relatively poor resolution of televisions and slightly the idea of refresh rates, nobody noticed interlacing.
Things changed with newer display technologies. An LCD monitor or television did away with refreshing the entire screen at very fast intervals. Instead, your image is produced by pixels that change individually. Partly as a result, interlacing became noticable.
Progressive video is the full video, without the deletions that interlacing entails. As a result it doesn't look strange and one could argue that more detail can be seen.
I'm at the end of what I had to say, but as I wrote this I realized how tired I am; I may have made some errors in my explanation. If you're curious about interlacing there's a good Wikipedia entry on it that I read up on when I started doing work with digital and analogue video (analogue video always had to be deinterlaced). Anyone who notices errors in my post, please reply with corrections.
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