It's all about controlling light.
When you are making an exposure you must assess how much light you have to work with, and what are the light capturing limits of your equipment. Whether you have your camera on auto or manual, aperture or shutter priority, each exposure you make will have some combination of f stop, shutter speed and ISO. Experience and understanding of how these 3 variables are set, are fundamental to gaining control of image sharpness, clarity, depth of field and desired exposure. How you juggle these three variables depends on the the circumstances of your subject and your artistic decisions.
For a digital camera, the ISO setting adjusts your light sensor's level of sensitivity to light. A higher ISO allows you to hand hold in low light situations. A lower ISO allows you to shoot in a bright situation with a wider aperture to throw the background more out of focus with a wider aperture. 1600 ISO and above, especially with older digital cameras, may give noise 'grain' and rainbow color speckles in the darker tones. Some of the newer high end DSLR's can produce clean, relatively noiseless images at 6400 and above. This is a new frontier in photography, allowing us to shoot in lower light levels than ever before. 100 or 200 ISO will give clean, clear, noise free image quality, with the best color and cleanest image quality. Do you mind the look of noise that comes with higher ISO? Or do you love the clean perfect colors a low ISO gives? Of course you could always use a flash in low light situations and set your camera to a lower ISO. But using a flash in a creative way to over ride or augment available light is typically an advanced skill. Direct flash is widely considered to be one of the least pleasing of light sources. The newer your camera is and the more you spent for it, the better the image at high ISO.
Check out http://www.dxomark.com/ to see how your sensor compares with other cameras and models at high ISOs.
Moving subjects may require a faster shutter speed to freeze action: Faster shutter speeds will freeze action and prevent lack of sharpness due to camera shake during exposure.
The longer the lens, the more danger you have of softness due to camera movement. Wide angle lenses are safer to use at slower shutter speeds than telephoto lenses.
A commonly used rule of thumb is use a shutter speed that is close your lens length, .e.g. for a 105 mm lens use 1/125 of a second, for a 200 mm lens use 250.
In my experience, a shutter speed of 125 is usually enough to hand hold my 200 mm or 400 mm zoom lenses. I routinely shoot at shutter speeds as low as an 8th with low light subjects that aren't moving, knowing I will have to throw out a lot of frames due to softness from camera movement during exposure. In our class, we routinely see students get sharp images at an 8th of a second. While this is possible, if your subject or you move during exposure, you will get some blur in your image. I find I can get a very high rate of sharpness at a 30th, unless the subject is moving during exposure.
f Stop / Aperture
Your aperture setting will control your depth of field. Depth of field determines the range of what's in focus in your image from the foreground to the background. Here's an image shot with the aperture set to 1.8, a setting that gives a very narrow depth of field. Notice how the eyes are in focus and the background and even her ears are out of focus. The narrow depth of field gives a softness that enhances and is appropriate to the subject.
Read more: Learn Photography Basics