What is iso and how is it used?

I currently use a Nikon d3x and a few other professional cameras.
I can never seem to figure out what iso is, i know what everything else is.
Can some one provide a detailed explanation?

You have a D3x & no idea what ISO is? Sheesh!

ISO is a measure of sensitivity of the sensor to light. It is equivalent to the ISO/ASA scale used in film cameras. Back then, you loaded a film with a set ISO and that was it until the roll was changed - the only parameters to change were shutter speed and aperture.

However, with digital, the sensitivity of the sensor can be changed between shots, so it's another parameter to adjust - part of the exposure triangle of aperture, shutter speed & ISO.

The higher it is set (more sensitive), the more light is registered. This means that higher shutter speeds can be achieved.

The downside to increasing the ISO is that digital 'noise' increases (seen as coloured speckles in the image).

Normally you should keep the ISO as low as possible, only increasing it as a last resort to get the shot.

If, for example, you were shooting sports indoors. You are using a zoom telephoto lens which doesn't have a very wide maximum aperture. Say your light meter is telling you a 'correct' exposure is;
ISO 200
shutter speed 1/60
aperture f5.6 (the widest this lens can go)

You want 1/250 sec to freeze the motion of the players, if you just increased the shutter speed without altering anything else, your image would be 2 stops underexposed
(1/60-->1/125-->1/250 is 2 stops less light)

so you need more light! You can't open the aperture any more, so the only other alternative is to increase the ISO by 2 stops - this will give you the correct exposure at 1/250 sec

(ISO 200-->400-->800 is 2 stops more light)

so your new settings would be;
ISO 800
shutter speed 1/250
aperture f5.6

This is the exposure triangle (ISO, aperture & shutter speed) one of the basic cornerstones of photography.

It's a measure of film's sensitivity to light, which has carried over into digital.

Generally, low ISO gives images greater clarity - at the expense of low shutter speed and wider apertures. High ISO creates degraded images (the higher the ISO, the greater the noise) but allows shutter speeds to be increased and/or the aperture to be stopped down. It also increases flash range.

The challenge is to balance all three variables to get the result you want.

You have a $8, 000 camera and don't know the basics of photography? How did that happen?

Here is a link that shows you how to balance ISO, shutter speed and lens aperture, but frankly, until you learn the fundamentals of photography, you will never get full use of your fine camera.

This situation proves that you can put your money down and by amazing camera, but you can't buy ten years of experience

ISO is a standardized expression of the sensitivity of a light reactive medium. That sounds a bit of a mouth-full but the idea is quite simple. It's there so that you can have a variety of equipment and always gauge how it will perform in combination with other equipment. Such that whatever equipment you have an image taken at (for example) f/4, 1/60s, ISO 100 in the same conditions will have the same light levels (roughly) even on very different equipment. So say you have two digital cameras, but one of them has a sensor that is twice as sensitive as the other, then the electronic gain on the more sensitive camera would be roughly half that of the lesser sensitive camera at ISO 100.

It's hard to explain this properly without getting too technical.

An ISO image (International Organization for Standardization) is an archive file (also known as a disk image) of an optical disc, composed of the data contents of every written sector of an optical disc, including the optical disc file system. ISO images can be edited by certain tools, like WinISO, ultraiso, etc.
Get more information: http://en.wikipedia.org/.../ISO_image