When shooting in manual mode, meaning having full control over an image’s exposure, there are three things that are directly related to each other: Shutter Speed, Aperture, and ISO.
ISO is the camera’s level of sensitivity to light. In the days before digital ISO (or ASA) referred to the “speed”, or how quickly light is absorbed. The higher the number the faster the absorption rate. As the number doubles so does the speed of light absorption: ISO 200 is twice as fast as 100, 400 is four times as fast, 800 is 8 times as fast, and so on. Now swap the piece of film for an image sensor, and the numbers carry over in the same way.
When to use a “slow” speed vs. a “fast” speed:
If you’re in a situation where there is a lot of light, you’re going to want a low ISO. A couple examples might be while you’re playing at the beach, or skiing down a snow covered hill in the middle of a cloudless day. In these situations you are going to have a ton of light both coming from the sun, and reflected off the ground. Be careful though, this kind of light can trick the camera into thinking there’s more light than there actually is.
In order to use your camera in low light situations, like indoors, without the use of a tripod or flash, you may need to boost the ISO. Be careful though, high ISO comes at a cost. The higher you go, the more noise and grain gets introduced into the image, and degrades the quality. Because of this, I will sometimes limit my ISO to something like 800 for the highest quality.
Here is an example of what camera noise can look like at a high ISO setting.
In the days of film, as soon as you put a roll of ISO 100 into your camera, you were stuck at that speed for the entire roll. The digital image sensors of today allow you the ability to change as you see fit. You could be shooting at ISO 100 outside for one frame, and a minute later be shooting at ISO 3200 in room lit by a single candle.
There is a bit of confusion as to where the term ISO comes from, and its pronunciation. A lot of people say, because you’ll always find it in all caps, it’s an acronym for “International Standards Organization”, and pronounce it “eye-es-oh”. This is actually wrong. There’s no such thing as the “International Standards Organization.” There is however the International Organization for Standardization, which sets numbers for all kinds of industries like engineering and manufacturing globally. So why isn’t it IOS? Instead of choosing an acronym to represent the company, they decided to use a word. ISO comes from the Greek word ISOS, which means “equal”. Therefore the correct pronunciation is “eye-so”.