How does changing ISO change your pictures?

Just got a new DSLR and I've kept it on Auto ISO and I've wanted to try changing it up. How does ISO affect the picture?How does changing ISO change your pictures?
ISO is Light Sensitivity relative to the image or subject matter you wish to capture. Current technology suggests that noise will result in the higher ISO levels but as that same technology improves with noise handling, the ISO will also deal better with noise.



I don't want you to think that fast ISO settings are going to give you lower quality images because, as the technology improves, that answer will also degrade accordingly.



Noise a the present time is an issue with some sensors and, as improvements are made in equipment the noise will diminish accordingly so higher ISO images will ultimately be as good as lower ISO images in quality.



Currently however... if you elect to chose a higher ISO in order to speed up your shutter speed, you risk the chance of a lower quality (not clarity) of image output.



Everything is give and take... in order to get speed, you surrender depth of field (wider aperture along with subject to lens distance) and, sometimes the sacrifice is accommodated by changing the ISO of the digital capture in order to satisfy the needs of the photographer.



Photography is making decisions based on your need to provide information that relays your emotional impression.

..How does changing ISO change your pictures?
Elaborating on what Anne has already said:

What ISO denotes is how sensitive the image sensor is to the amount of light present. The higher the ISO, the more sensitive the image sensor and therefore the possibility to take pictures in low-light situations. When you increase the ISO you only increase the sensitivity of the sensor, so that the same amount of light becomes more effective.
In an analogue camera, the ISO is the 'speed' of the film. it correlates to how sensitive (reactive) the film is to light hitting it.



HIgh ISO film (over 400) is used to either capture high speed events without blur in bright or normal light, or to take photos in low light. High ISO shots tend to be more grainy than low ISO.



ISO is combined with shutter speed and f-stop to get a correct exposure. If you have a bright light source and a low ISO, you need a slower shutter speed, and/or a more open aperture (f-stop) to get a properly exposed shot, than if you are using a higher ISO.



Digital SLRs mimic analogue SLRs, so if you treat the ISO as you would for a film camera, you should be able to play with different settings, shutter speeds, and depths of field to create interesting images.



**Note that ISO and shutter speed are not the same thing. They work together, along with f-stop to create artistic images for people who know how to use them. Playing with your camera is a good thing to do.**
ISO is used in dark environments. It helps to make photos brighter in dark atmospheres. However turning the ISO way up can add lots of noise to your photos.
Your best quality pictures will be at a low ISO. As you increase the ISO, you can take pictures in less light, but you are adding noise to the images. Your best bet is to only increase the ISO when it's necessary to get a proper exposure in low light.
ISO is the shutterspeed of a camera (how fast the camera opens to intake the light for a picture)

Some of the reasons photographers use a higher ISO is for action scenes ( sports ) or when the object of the picture is moving. The higher the ISO the sharper the image will be and vice versa.



hope that helps