The term standard television, or SDTV, only became relevant when high definition television entered the scene--before that, all TVs were standard, and so the specifications for this format were considered normal. The resolution for standard television is the comparative base for judging all other specifications, and anything beyond it is considered high definition.
Talk of standard television became more relevant when broadcasters started preparing for digital television. Before, analog signals didn't have as much variety in resolution because of physical limitations--digital formatting means resolutions can be changed on the fly.
Standard resolutions were conceived by several groups, based on older NTSC and PAL agreements. SDTV forms the baseline for determining if something is HDTV, that's why it's important.
Anything up to 640X480 is considered SDTV, or regular TV. This means 640 pixels by 480 pixels, interlaced. In other words, the screen is rendered in two halves, each 320X240 in size, which is the original screen resolution for NTSC. PAL broadcasts are slightly bigger. Remember both NTSC and PAL are analog standards; the term SDTV now refers to digital broadcasts.
Enhanced definition TV
EDTV is a relative of SDTV, expanding on it to 876X480. This larger image size was initially meant to be the base HDTV resolution, but was soon replaced by 720p, or 1280X720 progressive scan. EDTV was never used on analog sets, it's a digital resolution. Analogs never went beyond 640X480.
Because of the switch to digital television in June 2009, it's vital consumers understand the difference in terms--once more, SDTV refers to the lowest resolutions supported by digital TV, or 640X480. Regular TV and analog TV describe the normal broadcast resolution prior to the advent of digital television--also 640X480. All HDTV video is digital by default.
All free to air broadcasts post-June 2009 in the U.S are SDTV, with resolutions of up to 640X480 interlaced. Cable and satellite providers also use this resolution as their entry level, with higher resolutions, or HDTV, typically costing extra to receive.
To easiest way to gauge screen size is to regard resolutions as math equations. For example, 640X480 equals 307,200 pixels on screen at once, while 1920X720 means 1,382,400 pixels. This makes it easy to understand which of the formats provides a bigger, more detailed image. Keep in mind analog TV was originally 320X240, which is tiny at 76,800 dots.
Digital TV resolutions are backwards compatible. This means a 32-inch LCD can display both its native 720p resolution and anything below it, including standard definition TV. However, resolutions smaller than a screen's native size entail upscaling or stretching, detracting from image quality.
Old tube televisions don't have limitations on resolution, but analog signals can't carry sufficient data to support HDTV. Hence, while with digital TV the screen is the bottleneck, with analog TV it's the signal that's limited to certain resolutions. This is why an old TV can't show new HDTV signals.