Now I'm confused. I understand ISO & sensitivity to light, ( & need for a tirpod) but I guess I've been wrong in my thinking as far as dark situations. What would be a norm for a room as I said? My confusion comes in because I've been told to use 400 ISO & up in these situations.
You should adjust your ISO as you see necessary. If you are in a low-light area, trying to take a photograph without a tripod, you will most likely need to bump your ISO up so that your shutter speed is fast enough so that you can shoot the image hand-held without worrying about camera shake.
When doing long exposure/shutter speeds, you want to put the ISO at the lowest (i.e. 100) so that during the _____ amount of time that the shutter is open, noise sensitivity is as low as possible. This helps with the image quality and the amount of noise present when the shot is taken.
So for example, if you are in a room and you have your ISO set at 100. You might be getting an exposure value of 1/5th of a second at f/5.6 (example). This is quite risky to shoot hand-held. Even with a VR/Image Stabilization system active. So you bump your ISO up to 400. Now you get a exposure value of 1/30th of a second at f/5.6. Much better to shoot hand-held! This is also great if something is moving. At 1/5th, almost everything that moves will be blurred. Now 1/30th is still a bit slow for moving objects, but you get the idea....
So to sum it up, when doing long exposures, keep your ISO at 100 so that your image does not get too grainy due to more light hitting the sensor.
When you need a faster shutter in low-light, bump your ISO up a bit. This might cause some loss in image quality, but at least you will have the image, no?
When outdoors, you will usually have a sufficient amount of light to shoot with. So I suggest keeping your ISO at around 100.
It all comes down to your subject, and your lighting. Just remember to read your shutter speed, and see what you need to do.