A camera's image sensor is composed of tiny little electric light sensors. When you open your shutter, millions (billions) of light photon hits each individual sensor and the sensor gets excited goes to work using electricty to record the details of that photon. The camera's computer then takes all of the information it got from each sensor and puts it together to form a picture.
Due the the simple nature of electricity, exciting each sensor causes noise for the other sensors. The "noise" being almost exactly what you think it is. It disturbs the other sensors near it. This causes the sensor to not record exactly what hit it resulting in a slightly off version of the original. For example if orange is represented by the number 5 and the light scale from dark to light is 1-10. If orange light strikes the image sensor, due to noise, it may see the value which should be 5,2 (pure orage, very light) it records it as 4.7, 2.6 or a little off ornage and not quite a light.
Think about it like a giant gymnasium where there are hundreds of artists standing in the gym with a piece of paper. The powers above open the roof of the gym and each artists goes to work drawing on the sheat of paper only what they see exactly over their head. As soon as the roof is removed, they all go to work.... the room becomes loud, arms start flailing around, people bump into each other and not every artist is able to draw exactly what they saw. P&S and bridge cameras use a smaller gym, fewer artists, and they are all packed closer together, so its much more crowded and there is much more noise. DSLR's the gym is large, they have more artists, and they are all spaced further apart. This is why noise is inherently worse on point and shoots and bridge cameras then DSLR's
Now enter ISO.... ISO is basically an electronic amplifier. In order to make a picture better exposed you bump up the ISO, which literally amplifies the signal. So if a sensor records an light orange (a number 5) doubling the ISO would result in an image of 10. Now consider all of the pixels doing the same thing next to each other. If all of the pixels were recorded perfectly you would have an area where it would all look the same 5,5,5,5... a nice and smothe orange, if you amplify by doubling it 10,10,10,10 you still have a nice smooth orange. However, as we already mentioned, there is natural noise, so at your lowest ISO the image would be recorded at 4, 5, 5, 4, 6.... all realtivly the same and all looking the relativly the same.... but if you doublt the ISO and amplify the image you double the values and you get 8, 10, 10, 8, 12... instead of only being 1 number off, they are now two... .jack your ISO from 100 to 1600, doubling 4 times.. your numbers go to 16, 20, 20 , 16, 24. Now the numbers are way off and you can notice an obvious difference between the individual pixels.
Maybe this helps to make more sense... maybe it makes it more difficult. But it does explain why you are seeing the noise in your gray areas with your bridge camera.
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