A higher ISO can lead to image "noise." Noise is the graininess of an image that results from the image sensor from becoming "contaminated."* The larger an image sensor (a buying point to consider) you have, the more then this can be minimized. DSLR's, generally speaking, have an advantage at this because of their larger sensors. Also certain post-processing tools on software's like Photoshop can also drastically reduce the problem.
2. For outdoor shooting and in bright scenes, lower your ISO to its minimum settings for the highest quality. High ISO will generally be unneeded, because other settings can make up for any lighting problems.
3. For low light situations I use ISO as a last resort. When ISO needs to be utilized, I experiment with the lowest possible setting. For amateur shooters I suggest making your camera decide upon ISO automatically, by putting it on auto.
4. The larger the camera sensor the less likely noise will appear in your images.
So next time you go out to photograph consider ISO.
*People's approach to image noise differs because its based on personal preference. Back in the days when film was prevalent, people selected film rolls based on the the extent of grainyness in the shots they developed. This is because people use grain as an artistic tool while others find it undesirable.