terminology - What is ISO and how does it affect my photos

What is the ISO setting on my camera? What does it represent? What should I be aware of with regards to ISO when taking photographs

Answer

Changing the ISO setting in digital camera's is like changing the gain (amplification) of your photo sensors. With a low ISO setting (e.g. 100) you don't amplify the signal from the sensors. But using a high ISO setting (e.g. 1600) results in an amplification of the signals registered by the sensors. A high ISO setting usually results in more noise in your images so try to keep the ISO as low as possible.

This article explains various sources of image noise: http://en.wikipedia.org/wiki/Image_noise