What is ISO?
Changing your ISO allows you to adjust the amount of light recorded by your camera (known technically as ‘sensitivity’). If you’re shooting on a bright day, then you don’t need a high ISO number, because your camera has enough light to take a sharp image, but if you want to capture an image at night, then you would need to raise your ISO to maintain the same shutter speed and aperture.
General Use
Though we'd all love to use a high ISO to make our cameras more sensitive to light, there are drawbacks to ISO settings like 3200 or 6400. One of these is grain, known in photographic circles as 'noise'. For casual photography, noise isn't a major issue as long as you avoid tricky lighting conditions. If you're a fan of low-light photography, or if you need very fast shutter speeds for your photos, you may want to invest in a camera that performs well at high ISOs or a wide-aperture lens that allows you to let more light enter the camera, before you even get to ISO.
Sports Photography
Professional sports photographers need to use high ISO settings like 6400, because they take photos at dark stadiums or at night. Your photography is likely more casual, but the next time you consider a sports image, try focusing on your ISO setting - it'll make a big difference to your photography.
Wildlife Photography
Although wildlife photography is very similar to sports photography in most respects, I gave it a special mention, because you're probably going to be taking images in the day. I'd avoid your highest setting and go for ISO 1600, or a similar mid-range setting.
Landscape Photography
Landscape photography requires you to use as low an ISO as possible. Because your camera will be supported with a tripod, you don't need to use higher ISO numbers for a fast shutter speed, and can instead prioritize the elimination of noise.
Macro Photography
Macro, or close-up photography, often requires the use of specialist lights and lenses, and fast shutter speeds are crucial. Though you'll probably be shooting your images on a tripod, when you're working on a near-microscopic scale, a crisp image is your #1 priority.
Food Photography
Food photography will often take place in a dark, atmospheric restaurant, and so a medium to high ISO is needed to capture sharp images. You can avoid this by using your flash or shooting on a bright day.
Street & Urban Photography
Street photography will generally occur in the middle of the day, so a high ISO isn't always needed. ISO 400 should be perfectly adequate in most cases, but it's important to adjust this as needed.
Portrait Photography
The ISO that you should set for portraits is massively dependent on lighting conditions, but since you'll likely be in shade or outside, try anywhere between ISO 400 and ISO 1600. If you have a flash, then you can try ISO 200 or even 100.