It's been a long while since I've posted any photography/camera tips, so I figured I'd talk about one side of the exposure triangle - ISO!
ISO is basically how sensitive your sensor or film (in a film camera) is to light. Do you remember WAAAAY back when you used to buy film and you'd have to choose between, say, a 100, 200, 400, or 800 speed film? That was the ISO speed. Typically, the lower the ISO number, the less sensitive to light it is. And the higher the ISO, the more sensitive to light it is. We are lucky in the digital age to have the option to choose a different ISO with every shot, if we want. We aren't limited to one ISO for an entire shoot, like we would be on film.
A typical ISO range is 100 to 1600. ISO is the means by which you tell your camera how sensitive do you want your sensor to be.
So, in lowlight situations (say, inside at a wedding reception for example), you will probably want your sensor to be ultra-sensitive to pick up what little light there is. And conversely, a sunny day on the beach would call for the lowest ISO number available for your camera.
There is one very important point to keep in mind regarding high ISO settings and digital noise. The more sensitive you tell your sensor to be (ISO 1600, for example) the more susceptible is your image to "digital noise," which is an indication of "graininess" in your final image.
(Image from http://kernesti.com/Camera/basic.htm)
Some cameras can handle the noise factor better than others. Some cameras will put out ultra grainy pictures with a 400 ISO - not typical, but happens. Others can shoot at a higher ISO and put out less noise.
Can I give you a tip for less grainy photos at higher ISOs? Nail the exposure! If you create a correct exposure, even with a higher ISO, you'll get less grain in your image.
So, how does ISO work together with shutter speed and aperture? Well, if you want to use a faster shutter speed, and your f-stop is as low as it can go to let in more light, but it's still not enough light to create a good exposure? You can bump up the ISO.
That's just one example.
For me, usually what I do is determine what kind of light I'm dealing with, set my ISO accordingly FIRST. Then I decide if I'm dealing with an aperture picture (where I'm more concerned about depth of field) or a shutter speed picture (where I'm more concerned with stopping/blurring motion). I set the aperture or the shutter speed to the setting I want, then adjust the other until I get a correct exposure reading (meaning in your viewfinder, the little indicator is in the middle where the arrow is, like so:)
(where the yellow bar is right in the middle, it's a correct exposure)
Even many point and shoot cameras have the option to change your ISO, so get out your camera, your manual, and try it out! If you find you're getting too many blurry indoor shots, try bumping up your ISO! Have fun playing, and if you have any questions, feel free to ask!