Resolution
Format Resolution Description
TV ~500x480 Interlaced
VCD 352x240 progressive or interlaced
SVCD 480x480 progressive or interlaced
DVD 720x480 progressive or interlaced
DV 720x480 true interlaced
HDTV 720x480p progressive (technically not hdtv--this is dvd on hdtv)
1280x720p progressive
1920x1080i interlaced
Resolution, in rough terms, is a measurement of how much information there is in a given image. It's measured in dots (or pixels) horizontal by vertical. For instance, a resolution of 640x480 means a picture is comprised of a grid of dots 640 wide by 480 high. This yields a total of 307200 pixels, or, as you may recognize from the digital camera world, about 0.3 MegaPixels (MP). Likewise, a 3072x2048 image would be just over 6 million pixels, or, 6.2 MP. The higher the resolution, the more detail there is. If you take a picture at 640x480, then take the same picture at 3072x2048, the higher resolution image will let you zoom in a whole lot more and/or blow the image up a lot more when you print it without it becoming blurry.
Analog Television
Broadcast television, being analog, has a vertical resolution defined by how many scanlines are used (480), but doesn't really have a horizontal resolution. However, we can determine from calculations based on bandwidth approximately what the horizontal resolution should be. A television signal is squeezed into 6 MHz of bandwidth and, after much boring math, we are led to a horizontal resolution of approximately 500 pixels. This means we can define a broadcast television signal as having about 500x480 pixels or so. (Technically, television has not 480 but 525 lines, however a large block of those lines aren't actually on the screen and so we have only 480 visible horizontal lines.) It's important to also note that although TV is broadcast in a 6 MHz band, the television itself may be capable of considerably better resolution, depending on the quality of its screen. This is why, as we'll see later, it's worthwhile for DVD to be quite a bit better than just 500 pixels.
VCD
The first common digital format is the Video CD, or VCD. These used mpeg1 for compression and CD's as their media. Given the limitation of only 700 megs for a CD and the limits of mpeg1 compression, resolution is capped at 352x240 for VCD's. The resulting quality is about that of a VHS tape, though unlike VHS, it will not degrade over time and can be skipped directly to any chapter just as an audio CD can go directly to any track.
SVCD
A little later came the Super VCD, or SVCD. These use a resolution of 480x480. Thanks to the superior compression available from mpeg2, SVCD's double the vertical resolution of VCD restoring it to the highest quality capable on a standard television. Horizontal resolution has also been bumped up to 480, which is also very near the quality of broadcast television (about 500x480).
DVD
Finally, there is DVD which also uses mpeg2. Resolution has been increased to 720x480 because DVD's hold about 4500 megs compared to CD's. Vertical resolution remains at the limit of what a standard television can handle, and horizontal is a lot more detailed at a nice 720 pixels.
DV
DV tapes are a standard format for modern, digital camcorders. Resolution is also 720x480, same as DVD, but they only record in interlaced format. There are Progressive DV camcorders out there, but they're rare and expensive (but very nice). DV tapes use their own file format called (wait for it...) DV.
HDTV and the Infinite Beyond
With the advent of High Definition Television, we are finally able to break out of the 480 vertical we've been stuck in since the dawn of television over 60 years ago. Along with higher resolutions comes a new shape (see the essay on Aspect Ratio). HDTV format is a 16:9 rectangle--considerably wider than the standard 4:3 television. HD broadcasts come in two flavors - 1280x720p and 1920x1080i (and soon 1920x1080p). Not only can HDTV's receive these broadcasts in all their high definition glory, they are also capable of playing DVD's progressively (which is something a standard television is not capable of)--this is the so-called 480p format, though it is technically not an HDTV format. See the essay on Interlacing for explanations of progressive and interlaced.