ISO speed tells you how sensitive the image sensor in your camera (back in film days, the film) is to the amount of light available. So, the higher the ISO, the more sensitive the image sensor(film) is. If you are using a high ISO, you can take a picture when there is not a lot of light available. Beware though, the higher the ISO, the more noise (grain) is seen in your photo. Noise is like artifact almost in your picture, your picture will just not look as clear with a higher ISO than a lower one. If you use a low ISO, the less sensitive the image sensor (film) is to light. Then you would want to use a low ISO in the afternoon on a sunny day. What is great about digital cameras is that we can change our ISO anytime we want depending on the light available. Back when and still when people use film, they have to change the roll of film to work with their lighting situation.
One site I found www.great-landscape-photography.com/iso-speed.html stated that an ISO of 50 is good for bright sunny days, and maybe even an ISO of 100 would be okay also. For low-light conditions like late afternoon, early evening an ISO of 400 would be good. Then for really low light-light conditions like night time an faster ISO of 1600 or 3200 would be good, but to expect noise. These ISO values were a little different at a different site listed below. The only way to decide which ISO you will like for certain types of the day is to practice shooting all different ways.
The other night when I was taking the metering photos of the candles I used an ISO of 200, which is my camera's default setting. I think that tonight I am going to try a higher ISO and see what happens. I think the exposure was good, but I had a little bit of blur just because of the long shutter speed. If I wasn't so darn lazy I could of pulled out my tripod and set it up, and then used my self timer on my camera to ensure no camera shake, but that was too much work for me at that time. So, if I raised the light sensitivity (ISO) of my image sensor the shutter speed could be faster and create a more crisp image...we will see. However, I might get some noise though. I will post the pictures later tonight. I read an article by Bryan F. Peterson about ISO and worker bee's, which made it easier to understand ISO and shutter speed. He said that if you have you ISO set and 100 and I had mine set at ISO 200, and we both had our apertures set at F/5.6 that I would take my picture faster than you! Why? Because I have 200 worker bee's trying to gather the light and you only have 100. Neat.
Here are a list of ISO's and times of day to use as a reference created by
ISO 50 for bright light/bright overcast
ISO 100 for heavy overcast/heavy shade
ISO 200 for early morning/late evening
ISO 400 for dawn or dusk
Use flash at night or up ISO as mentioned below. I also read that if you need to increase shutter speed even more, like to catch your toddler running after something, you can up your ISO to 400 and this will amp up your shutter speed, even if you have enough light available at 200. I guess I will have to give this a try, I wonder if it would overexpose the picture with too much light. Though, the author at PictureCorrect.com did say that this could increase your noise.
Try not to use flash, and try upping your ISO. First you will need to open your aperture all the way, and this you can make your shutter faster, and even faster with that higher ISO. For every F/stop lowered (bigger aperture) you can increase your shutter speed by half. Each time you double up your ISO (example 200 to 400) you can halve your shutter speed (from 1/2 second to 1/4 of a second). This will help you not use flash and not have camera shake due to a long shutter speed.