In very simple words, ISO is the level of sensitivity of your camera to available light. The lower you set your ISO to be the less sensitive it becomes to light, and off course with higher ISO, the sensitivity increases too!
When to use what ISO?
When there is enough light, use a low ISO to get the shot with the highest detail possible! More ISO will allow you to get more light in the shot, you don’t want too much light as it will decrease the details!
High or low ISO?
Well, it really depends. Indoors, outdoors? Flash, no flash? Personally, when I am shooting indoors I will go and use a high ISO given that I am not using a flash! This just allows me to capture the shot properly without having any or minimal blur to the image.
Want to capture something moving fast?
Like that Lamborghini or the president’s copter? Well in these cases you might use high ISO just to allow enough light for the details. But remember the higher the ISO the less the detail, so before deciding a high ISO for your Lamborghini. Do not forget you will have noise due to the increase in the ISO.