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To understand the resolution nomenclature, you need to understand the difference between progressive scan and interlaced video. Progressive scan video is scanned from side to side, top to bottom: line one, then line two, line three, and so on, up to the end of the frame. Interlaced video is scanned from side to side, top to bottom, as well. The difference is that every 60th of a second, every other line making up the complete frame is scanned. Then, a 60th of a second later, the lines in-between the lines that have already appeared are scanned. Effectively, half the picture’s vertical resolution is sent in the first 60th of a second, and the second half is sent in the second 60th of a second. When the video is played back, the whole thing happens in reverse, giving the appearance of a complete frame. Each of these “halves” of a frame is called a “field.” If you were to look at an interlaced picture on a screen for a 60th of a second, you’d only see half the vertical resolution — every other line. But, because our eyes and brains conveniently offer a “persistence” effect, when you look at the screen “normally,” what you actually see is something approaching full resolution.

The bottom line is that progressive scan has about about 50% more vertical resolution than interlaced pictures with the same number of lines, and does not show the aliasing artifacts associated with interlaced video.

In marketing materials, progressive and interlaced video are abbreviated p and i respectively. They are usually preceded by a numeral, which is the number of pixels (or lines) tall the image is. Since the typical screen aspect ratio is 16:9, you can figure out the width of the image accordingly.

1080p

The highest resolution currently available in consumer HDTVs, 1080p is 1920 pixels wide by 1080 pixels tall, progressively scanned. This is the primary resolution for Blu-Ray and HD-DVD. It is also supported by the PS3. Aside from those devices, virtually no 1080p content exists. There are no 1080p TV broadcasts at this time, and probably won’t be in the near future. 1080p HDTVs are becoming fairly common in consumer stores, but the technology is still quite a bit more expensive than its 1080i counterpart, especially when you consider the lack of content available. I personally decided against 1080p at this time — it’s great technology, but to me, the quality difference wasn’t worth the price, especially when I don’t see myself owning a Blu-Ray player or PS3 anytime soon.

1080i

1080i, which is 1920 pixels wide by 1080 interlaced, is the highest resolution that there are currently TV broadcasts available in. Many TV networks are currently broadcasting in 1080i, and it is the highest resolution found on the typical HDTV set (excepting those, of course, that support 1080p). While 1080i technically offers the same pixel resolution as 1080p, the interlacing results in a lower-quality picture.

720p

A very common HDTV resolution, 720p is 1280 pixels wide by 720 pixels tall. When compared to 1080i, some will assert that it offers a better picture, due to its progressive scan nature. Some TV networks, including those that broadcast a good deal o sports (ESPN, ABC, etc.) have settled on 720p instead of 1080i, as it provides a smoother picture with the fast motion of these types of broadcasts. Ultimately, you don’t have to decide between 720p and 1080i, because virtually every TV that supports one supports the other. 720p is the lowest resolution that is considered high-definition.

576p

Often referred to as EDTV, or enhanced definition television by marketers, 576p is a resolution in between standard-definition and high-definition. Usually 720 pixels wide and 576 pixels tall, this is a common resolution for DVD Videos using the PAL standard popular in other parts of the world (besides America).

480p

Also referred to as EDTV, 480p is 720 pixels wide by 480 pixels tall (16:9), or 640 pixels tall and 480 pixels wide in the case of 4:3 aspect ratio content — progressively scanned. This is the common resolution for DVD Video. It is also the resolution of videos available from the iTunes store. All modern HDTVs support 480p.

480i

Standard definition television (what we’re used to, and what you see if you don’t have an HDTV), is broadcast in 480i, or 640 pixels wide by 480 pixels tall, interlaced.

QVGA

Although not found in TVs, it seems worthwhile to mention QVGA, which is 320 pixel wide by 240 pixel tall video. It is a common resolution for downloadable video, and was the original resolution for iTunes video.