Great advise Rosic! I too used to make my living as a photog...
Without being to technical, their is one thing I'd like to ad that may help someone avoid a pitfall...
When taking pictures in low light situations it is critical to understand "depth-of_field" and "shutter speed". Take the window lighting situation for example... unless it is fairly bright window light, you will be forced to "open your lens" or use a slow shutter speed in order to obtain a correct exposure.
When you open your lens, what you are doing is changing your cameras f-stop setting to a smaller number to allow more light to pass through the opening (iris) in the lens. This action makes the lens opening larger. The trade off here is that your "depth of field" will be reduced... which means the plane of focus becomes shallow. This becomes especially critical if you're photographing two or more subjects... One subject might be in focus and the other, out of focus. Depending on how low the lighting situation this can ruin a single subject as well... it's possible for his/her face plane to be focused and her ears and clothing to be out of focus.
This brings me to my next point... "shutter speed". To compensate for low lighting one might also choose to use a slow shutter speed to obtain a correct exposure. One of the pitfalls with this situation is that when photographing children they tend to move alot, which will result in a blurred subject... even if you use a tripod. The photographer has to be steady and the subject relatively still.
When using a camera on auto, the camera makes these two critical decisions for you, and you might not even be aware that the camera is struggling to compensate for low light and the result could be blurry images.
Sorry to sound so technical, but I don't know how else to explain my point. The bottom line is to shoot in "bright" shade or fairly bright window light if you are not using strobes or bright artificial lighting. If you have a choice shoot in the brighter (not harsh) areas.
In ideal situations, you'll normally want your shutter speed at a 60th of a second or faster. Also, try to keep your f-stop at f8 or higher.
Just to add a technique to the discussion... If you find yourself with only bright sunlight, have someone hold a white sheet between the sun and your subject... the diffused lighting from this situation is very pleasing. Their is professional diffusion material that you can buy, but a sheet works just as well.
Sorry to put you all to sleep.