In the days before digital photography, seemingly every corner store had rack upon rack of film on display. Each roll of film was marked with a speed--measured in ISO--such as 100, 200, or 400. Higher-speed film was handy for low-light photography, but it had a serious disadvantage: grain.
Film grain was every photographer's nemesis. Instead of smooth, natural textures, grain put ugly blotches all over a photo. And though the days of grainy photos are far behind us, digital photos have a similar problem: digital noise.
You've undoubtedly seen noise in your own photos. On the plus side, noise tends to be very small; and when you view a many-megapixel photo on a computer screen, pixel-size noise is so small that it usually disappears into the background. You might look at a very noisy photo and not even know it. Noise becomes apparent, though, when you zoom in--if you crop it down to a small detail, for example, or if you attempt to make a large print. Let's learn how to control noise.
The science of noise
Noise is an appropriate term, because it describes what happens visually in a photo: Tiny, fairly regular but unintended splotches of color mar the photo. The source of the problem is "noisy data"--unreliable or contaminated data. And that's what is going on here.
Millions of photosites crowd your camera's sensor, and each one registers incoming light as voltage. Unfortunately, photosites can get confused. If the light is too weak or the sensor's sensitivity is set too high, photosites may interpret random voltages as light. The result is a photo with noise.
Various factors can contribute to noise in photos. The one you have the least control over is the size of your camera's sensor. The smaller the sensor is, the more noise will appear in your photos, because the tightly packed photosites tend to overlap and interfere with each other. That's why smartphones, which have tiny sensors, tend to generate very noisy photos, regardless of any other factor. Compact digital cameras are somewhat less noisy, and full-frame digital SLRs--which have comparatively big sensors--tend to have the least noise. All other factors being equal, a DSLR will take a cleaner photo than a smartphone every time.
Controlling noise with ISO
Unlike sensor size, your camera's ISO setting--which controls the camera's sensitivity to light--is largely within your control. For example, you can increase the ISO setting to take sharper photos in low light. However, a higher ISO also means more noise. Of course, sometimes the trade-off makes sense. If you're trying to take photos late in the day without a flash, cranking the ISO is your best option. But when you're done, dial the ISO back down as low as it will go, so you can avoid taking needlessly noisy photos the next day when you're shooting in bright sunlight.
People often shoot at higher ISOs without realizing it. If you set your camera to Auto, for example, it will probably increase the ISO without telling you; that's a good reason to shoot in Program mode instead.
If you have a smartphone that you use for photography, you might have some control over ISO. Windows Phone and Android users, for example, can manually set ISO. The iPhone, however, has no ISO control--take a photo in low light, and the iPhone automatically increases ISO to try to compensate for those conditions.
Keep it cool
Camera sensors tend to generate more noise when they're hot. Thus, if you leave your camera on the dashboard of your car in blazing sunlight for a few hours and then try to take some photos, the sensor will be hot enough to generate significantly more noise, even at a low ISO. No matter what kind of camera you have--from an iPhone to a DSLR--it will take better photos if you keep it cool.
Likewise, you can raise a sensor's temperature by pushing its performance limits. The more you use the sensor, the hotter it gets. Specifically, your sensor can get quite toasty from taking very long exposures--a 30-second nighttime exposure is likely to be very noisy, for example--or from capturing images in burst mode, in which you shoot a dozen photos in just a few seconds. Probably the most noise-inducing thing you can do is to shoot video: If you record 5 minutes of video and then immediately start snapping still photos, those images will almost certainly suffer from a lot of noise.