How to use ISO in digital photo camera?

there is a part named ISO in menu of camera? how can i use this part and what is ISO 1000,800,200?

In basic terms ISO measures the light sensitivity of the camera's sensor. The camera if you set it on program mode, will automatically adjust the ISO so that you get a perfectly exposed picture. If you want to have manual control, you should know when to use which ISO settings. This is determined mostly by experience and general rules.

A general rule is that you want to use the lowest ISO possible, which will allow you to expose the picture in the given lighting condition accurately. Therefore, you would use a low ISO like 50, 80, 100, 125 in sunlight or bright outdoors. You would use a high ISO like 400, 800, 1600 for dark indoor lighting. For mixed lighting like on a cloudy day you can use something like 160, 200, 250, 320. Most of the time, you would shoot at ISO 100 to 400.

The purpose of not using a high ISO when not needed is that high ISO introduce noise into your pictures. This affects image quality and your pictures will appear grainy and not sharp when you zoom in. At a high ISO like 1600, you don't need to zoom in to see the degradation in image quality; it is very noticeable.

I don't want to get into the principles of ISO. It is mostly technical details, but if you are interested it is important to read about how it works in a film camera before looking at how it works in a digital camera.

you are better off using auto iso.
iso is the camera's sensitivity to light.
the higher the number - the grainier the picture

The ISO is like film speed in a film camera. You want to use the lowest ISO for the best picture but in lower light using a higher ISO will let you take a picure when you cant get close enuff for your flash to work . Example when it's cloudy maybe ISO 400 or inside. Really bright day 100 or 200 if you go into the higher settings like 800 and above you start to get noise that causes grainy looking pictures so only use it if you have to. Some cameras like my canon eos rebal xt has a noise filter that helps at those higher ISO settings. I was at an concert outside and the stage was way to far away for my flash to work and it was starting to get dark. My pictures were starting to get dark so I upped the ISO to 800 and then I could still get acceptable pics for a little while until it was to dark.

ISO; in film cameras it means the film rating sensitive to light.
The higher the number, the more sensitive to light it is.

-100; good for outdoors and flash
-200; good for outdoors and flash with some action
-400; good for outdoors, indoors, flash and fast action(sports)
-800; good for outdoors, indoors, flash, fast action (sports) and low lighting like a play performance, outdoors at night, etc.

Play with it and get to know the settings. Until then, you're better off setting it at auto ISO, letting the camera choose for you.

it means the light

Most consumer digicams have awful high ISO performance due to noise, which is much worse than film grain. Many p&s digicams are unuseable above ISO 200 .The mid and higher-level Fuji cameras do a better job controlling noise than most outside of a dSLR.

When I use a digicam, I set the ISO myself., and only use high ISO if there's no other way to make the capture.