Can someone explain what ISO is?

Shopping around for a new and my first digital SLR, I keep seeing the term "ISO" mentioned. ISO 200-3200. What does this mean or what is it used for?

Here you go from Wikipedia:

Film speed is the measure of a photographic film's sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system. Relatively insensitive film, with a correspondingly lower speed index requires more exposure to light to produce the same image density as a more sensitive film, and is thus commonly termed a slow film. Highly sensitive films are correspondingly termed fast films. A closely related ISO system is used to measure the sensitivity of digital imaging systems. In both digital and film photography, the reduction of exposure corresponding to use of higher sensitivities generally leads to reduced image quality (via coarser film grain or higher image noise of other types). Basically, the higher the film speed, the worse the photo quality.

Historic systems

The former American Standards Association (ASA) and the German Institute for Standardization (DIN) each promulgated film speed standards that have now been combined into the current ISO standard (see below).

The British Standards Institute produced their own scale. It was for all practical purposes identical to the DIN system except that the BS number was always 10 greater than the DIN number. It may be that similarity that lead to its rapid demise.

GOST (Russian: ГОСТ) is an arithmetic scale which was used in the former Soviet Union before 1987. It is almost identical to the ASA standard, having been based on a speed point at a density 0.2 above base plus fog, as opposed to the ASA's 0.1.[1] After 1987, the GOST scale was aligned to the ISO scale. GOST markings are only found on pre-1987 photographic equipment (film, cameras, lightmeters, etc.) of Soviet Union manufacture[2].
[edit] Current ISO system

The current International Standard for measuring the speed of color negative film is called ISO 5800:1987[3] from the International Organization for Standardization (ISO). Related standards ISO 6:1993[4] and ISO 2240:2003[5] define scales for speeds of black-and-white negative film and color reversal film. This system defines both an arithmetic and a logarithmic scale, combining the previously separate ASA and DIN systems.[6]

In the ISO arithmetic scale, corresponding to the ASA system, a doubling of the sensitivity of a film requires a doubling of the numerical film speed value. In the ISO logarithmic scale, which corresponds to the DIN scale, adding 3° to the numerical value that designates the film speed constitutes a doubling of that value. For example, a film rated ISO 200/24° is twice as sensitive as a film rated ISO 100/21°.[6]

Commonly, the logarithmic speed is omitted, and only the arithmetic speed is given; for example, “ISO 100”.[7]

Here's the beauty of this. In the old days (and on my current film camera) you buy film that's 100, 200, 400, 800 and put the roll in your camera. That's it - you shoot at the speed of the film you've inserted. if you're going to be indoors with a flash - 200. Outdoor landscapes - 100?. More movement - toss up - 400. Action - 800 or even faster for running sports - race cars, etc.

Now - in digital cameras you can either set the speed for auto - or you can manually change the speed with every picture - not being tied into a whole roll of 200, 400, etc.

Have you been in photography long?

Yeah, I remember the film camera days!! It seem so much easier back then.

I was told that ISO is like worker bee's, I forget how that made sense, but it seemed to make sense when it was told to me!

Nice explanation Lucky One!! I sure miss the film days!! Now almost everything has gone digital and left me in the dust. I have a lot to learn.