ISO is one of the three fundamental exposure parameters and something you need to have mastered to take successful photos. If you are new to photography and still working on learning the basics, this great video tutorial will show you everything you need to know about ISO.
Coming to you from Photo Genius, this excellent video tutorial will show you everything you need to know about working with ISO. If you are still learning the basics, you can generally think of ISO as your sensor's sensitivity to light — the higher the ISO, the less light you need to make an exposure. The tradeoff, however, is increased noise and decreased dynamic range, thus the need to balance your ISO settings with the other exposure parameters. One thing to remember, however, is that it is always preferable to have a sharp photo with noise than a blurry photo without noise. You can always take steps to reduce the noise in post-processing a bit, but once a photo is blurry because you used too slow of a shutter speed for either your subject's motion or to prevent camera shake, there is little that can be done to save it. Check out the video above for the full rundown.