When I got my first digital camera, I explored the menus, and I discovered there is a section called ISO. I didn't know what it was, but discovered the highest rated ISO was 1600. Not knowing what is ISO, I set it up as 1600. I got a lot of grains in the photos I took in low light. I thought my camera wasn't that good. Later I thought there was dust in front of my camera lens. I started hating my camera.
At that time I didn't know one thing. I didn't know what ISO iss.
ISO is comparable with Film Speed in the film cameras. Higher ISO ensures higher shutter speed, ensures reduced image quality, ensures more noise in the photos. You can think of ISO as light multiplication factor. An ISO rating of 200 means that it is twice as bright as the natural light that you could capture using ISO 100. ISO 400 means it is 4 times as bright as the natural light.
If you are shooting landscape, I will recommend you to use ISO 100. In the early morning, or late afternoon, you can try ISO 200. The less ISO, the better image quality. More ISO, worse image quality.
If I could, I would shoot everything in ISO 100. But I cannot. If I am shooting in low light, and the subject is moving too fast, higher ISO will ensure the sharpness of the photo. Otherwise, the image may get blurred.
Canon, Nikon and other camera companies these days support ISO more than 100000. That is meaningless. You may capture the face of your assassin hiding in your garden with that kind of ISO, but the photos will almost always never be good.
If you are shooting inside your room, keep ISO to 400. At max 800. Use large aperture. If you capturing one face, keep the f-stop in between 1. 8 to 2.8. If you are capturing many faces, use f-stop between 4.5 to 6. If people are moving, use flash.
Forget higher ISO. If you take care of ISO and use it effectively, even with a compact camera you will get stunning quality images.
Check this link. Here, a number of photos were taken by a compact camera. See if you can distinguish them.