ISO

Top picture taken at; F5.6, 1.3s, at ISO 100. No Grain in the photo, its sharp.

This photo was taken at F5.6, 1/13s at ISO 1600. This is as high as the ISO will go on my camera. You can see a lot more gain in this one, as apposed to the first picture. Especially in the background

F10, 1/30s at ISO 1600. Image is blown out and over exposed.

F10, 1/30s at ISO 800. Outside is over exposed, while inside the window ledge is correctly exposed.

F10, 1/30s at ISO 400. The outside is over exposed, while inside the window ledge is slightly under exposed.

F10, 1/30s at ISO 200. The outside is slightly over exposed, while inside the window ledge is under exposed.

F10, 1/30s at ISO 100. The outside is correctly exposed, while inside the window ledge is very under exposed.

ISO is how sensitive the is camera to light. The lower the ISO, the less sensitive the camera will be. This is best used when there is already a lot of light available, for example, outside on a sunny day. A higher ISO works as the opposite. The camera will be very sensitive to light, its best used when there is little light available, for example, indoors at a concert. This could enable you to take photos without the use of a tripod.
If your using a higher ISO, your likely to get more noise in your images. This causes your photos to be grainy and less sharp. In some instances the noise can add a nice texture to your image, it just depends on what type of look your going for. Noise is not very good in landscape photography, where most of the time you would like all of the image to be sharp and in focus.

The top images are to illustrate the noise you can get from different ISO settings. My camera only goes up to 1600 ISO, but you can still see some grain in the image.
The bottom images are to illustrate how sensitive the camera can be at different ISO settings. The photos were taken on a sunny day, from inside my house. I used all the different ISO settings i have on my camera.