What is ISO in photography?

ISO stands for the International Organisation for Standardization. What does this mean?  It's a standardization that was introduced to film rolls before the days of digital cameras, and refers to how sensitive the film was to light. So ISO is all about exposure to light and how bright or dark your photo will be.

Low light sensitivity film (low ISO) had to be exposed for a long time in order to get a photo with a reasonable amount of light registering, whilst high sensitivity film (high ISO) only needed to be exposed for a short duration.

It's pretty much the same thing with digital ISO, which you can control for every shot. Changing the ISO settings changes the camera's light sensor's sensitivity to light. The higher the ISO the more sensitive the sensor is to light and the brighter your photo will be.

ISO usually ranges from 100 to about 3200.

Benefits of low ISO
When should you use low ISO? What are the benefits of low ISO?

• It's handy for outdoor shots and shots where there is already sufficient light present. Using flash and/or other special lighting devices allows you to use a low ISO whilst maintaining enough light in your photos.   If you have low ISO and insufficient light, increasing aperture and exposure time also helps (i.e. set your camera to a slow shutter speed). You can also use a tripod to help you use slower shutter speeds to maintain a lower ISO.
• Low ISO gives better quality shots: The lower the ISO the better quality the image will be, so as a general rule of thumb, you should aim to use the lowest ISO possible. This enables you to blow up the picture to larger sizes and still maintain better quality. When you use high ISO, the picture is at risk of appearing grainy or speckled because ISO introduces "digital noise" to the photo. It generally creates an effect that isn't as clean and sharp as you'd perhaps like and take note that this graininess is often not perceivable when looking at your shots through the LCD screen after shooting, but it will be noticeable when you upload your pictures and scrutinize them on your computer. Good cameras should exhibit less digital noise, but even the best cameras probably have some at high ISOs.

Benefits of high ISO
When should you use high ISO? What are the benefits of high ISO?

• It's useful in indoor situations where there isn't as much light, or in generally darker situations where you need to boost light but aren't allowed to use a flash or lighting devices.
• If you want to create a grainy, speckled effect in your photos, you can use high ISO to achieve this. Some photos look better with grain, although there are post-editing programs that allow you to create this effect on photos too. 
• It's useful for when you need high shutter speeds and you need as much light sensitivity as possible without allowing for it with shutter speed exposure. High shutter speeds may be important if you're holding the camera on a very wobbly surface (e.g. from a moving vehicle), or if you're trying to photograph a still shot of a moving object.
• If you know you need a low quality, small image and grain doesn't matter, increasing ISO can be a quick-fix for getting the lighting right.