Back in the days of yore when everything was black and white, cameras used film. I.S.O. was a measure of the film's speed. The lower the I.S.O. the less sensitive the film was to light and vice versa. Now that we're living in the future, most folks use digital cameras rather than film. Digital cameras still have I.S.O. but now it measures the light sensitivity of a sensor instead of film.
Here's a quick video from E. Eilding that explains it even more!
So, now that we're clear on what I.S.O. is, let's see what the different I.S.O. settings look like! Leave it to the Vimeo community to upload tons of I.S.O. tests. Here's an especially good one by Amila C. Kumarasinghe. He uses a very low-light setting to take us from I.S.O. 100 to 6400 on his Canon 60D.
If you paid close attention, you'll see that the higher the I.S.O. the more light the camera picked up, but the image started getting noisy. Digital noise makes the color black look grainy and fuzzy (noisy!). Here's another test video from Andrew Schär that shows the noise in each of the different I.S.O. settings on a Canon 60D.
So what's the best way to use I.S.O.? Well, you'll typically want to choose the lowest I.S.O. that still gives you a good image. In low light situations, you'll have to bump it up but try to stay below 3200 if you can!
If you have a camera that allows you to change your I.S.O. you'll usually be able to access it through the menu and it will probably look something like this:
Good ISOs: 160, 320, 640, 1250, and 2500
Avoid using: 125, 250, 500, and 1000. These ISO settings create noise and make your footage look grainy.
Keep I.S.O. in mind when you're shooting as it's one of the more critical settings to check before you start shooting your awesome video.