This section is for photography rookies. If you know about shutter speed and the "shaky hands motionblur"-syndrome, then just skip this step.
So, you propably used a camera more than once, and maybe you have noticed, that when outside in the sun, the pictures are most often crisp and sharp, without any ugly noise.
When shooting in low light however, like indoors, you have propably noticed that the picture tends to get blurry and maybe with visible noise. Alot of people tend to believe it is because their camera "sucks", but often, it is because they just do not know the reason why these artifacts occur.
First I will try to explain the blur.
When you take a picture, you expose the sensor in the camera, so that light reflected from the subject, can hit the sensor and be registered as an electrical impulse.
This happens over a period of time. A longer period of exposure lets more light hit the sensor, which gives brighter images.
Unfortunately, during this exposure time, the subject might move compared to the camera.
Logically this will place the object in a new spot on the picture taken.
The object will therefore appear on the final picture both where it was at the start of the exposure, but also where it was at the end.
It will also leave a trace of itself between the two points, and looks transparent because of whatever being behind the subject at the beginning of the exposure, is visible to the camera at the end of the exposure.
For more info on motion blur, see here
The "shaky hands syndrome" is a result of motionblur, but not where the subject moves.
You might think you can hold the camera completely still, but you still move your hands (and the rest of your body) a tiny bit. Might not seam as much, but it will be alot for the camera,especially if you have zoomed in
This small movement will create motionblur. Not because the subjects move, but because the camera moves. When you hold your camera "still" your muscles jitter, and your balance shifts slightly, moving the camera slightly in several directions.
This makes it occur as general blurring, like it was out of focus.
The longer exposure times you use, the worse it will get.
Outside in the sun the exposure time (shutter speed) might be 1/125-1/1000 of a second, making the motion blur way too insignificant to be visible.
Inside however, you might only get 1/30, or maybe 1/2 second exposure time. This leaves plenty of room for motion blurring.
Modern cameras on automatic will try to avoid the long shutter speed.
To compensate for the smaller amount of light gathered, it either opens the aperture more, which will let more light through the lens, or, when the aperture can not get any bigger, make the sensor more sensitive.
The image sensor builds up a small charge at each cell depending on the amount of light hitting them. More light, bigger charge, brighter pixel.
(each 4 cells represent a pixel, with each cell registering blue, red, or green light, with the last one being used differently depending on sensor design).
Only problem is that these sensors are not perfect.
Alot of things can increase or decrease the charge at each cell. Temperature, difference in sensitivity between pixels etc. can make a each cell give off a too high or too low charge. This appears as too bright or too dark pixels of different colors on the final image.
The more sensitive the sensor is, the more noise there will be on the final image.
More info on the subject here: http://en.wikipedia.org/wiki/Image_noise
Fortunately for us, the noise pattern changes each time you take a new picture.
Phew! that was alot of work. hope you understood it :) Else, add comment, and I'll try to rephrase something, or add a picture for explanation.