ISO is another one of the three most important features (the others being aperture and shutter speed) that a photographer needs to know to help them take awesome shots.
Basically ISO is the level of sensitivity your camera has to light, meaning it measures how sensitive the image sensor on your camera is to light. The image sensor is the part of your camera that essentially takes light and transforms it into an image.
Unlike with aperture, ISO doesn't work backwards. The lower the ISO number is, the less sensitive your camera will be to light. The higher the ISO number, the more sensitive it will be to light.
You will want to use a higher ISO setting when you're in a darker environment, such as a gig or some indoor event. This allows you to capture images in low-light situations without having to use a flash. Therefore a lower ISO works best in lighter environments.
A very important thing that needs to be noted however, is that, as you crank up the ISO, you will get more grain or 'noise' on your image. So where you can (in environments with plenty of light) keep your ISO number low.
However it may be possible for you to use a low ISO in a dark setting. If your camera is sitting on a tripod or some other flat surface that will keep it absolutely still, then you could try a low ISO. This just means it will take your camera a longer period of time to actually capture the image, so both the camera, and anything in the scene, cannot move during that time if you want to capture it in perfect detail. Anything that does move while your camera is trying to take the shot will be blurred or might look a bit like a ghost. In some cases this can create cool effects, so maybe try playing around with this.
The lowest ISO number your camera has is known as the 'Base ISO'. For most cameras this is usually between 100 - 200. From here the numbers tend to go in sequence like this: 100, 200, 400, 800, 1600, 3200 and 6400. Each time you increase the ISO number you are basically doubling how sensitive your camera is to light. So when your using ISO 200, your camera is two more times sensitive to light, at ISO 400 it is four times more sensitive, at ISO 1600 it is sixteen times more sensitive, and so on.
Now apart from helping you take pictures in darker settings without a flash, what else does changing the ISO do for you?
Well, it will also help you take images of fast moving objects, because the higher your ISO, the less time your camera needs to capture what is in front of it. For example, when the camera is set at ISO 1600, this means your camera can capture an image in 1/16 of a second. As the ISO gets higher, the amount of motion blur in your image is reduced and a moving object can essentially be frozen in time, which allows you get capture more details of the object.
ISO 100 = 1 second
ISO 200 = 1/2 second
ISO 400 = 1/4 second
ISO 800 = 1/8 second
ISO 1600 = 1/16 second
ISO 3200 = 1/32 second
A great feature on many new cameras these days is called 'Auto ISO'. The name is pretty self-explanatory, it enables the camera to pick which ISO setting it thinks is best for the environment you are shooting in. What you can normally do in this function is set a maximum ISO, such as ISO 1600. This means the camera, when it is picking what ISO to use, will not be able to use anything over 1600.
While auto features are a brilliant helper, particularly if you are starting out, try not to get stuck constantly using them. That's what I hope these series of posts can do, give you the information you need to allow you to start experimenting with the settings yourself on manual.
Remember when you start playing around with any of the settings (aperture, ISO or shutter speed), you will also need to start adjusting the other settings accordingly. But that's another post for another day.