How does radioactive decay affect our lives?
It can cause cell mutation leading to diseases such as cancer
In the same manner it can also be used to cure cancer - when a large dose of radiation emitting waves are directed to cancerous cell
And it is also used it food processing
Radioactive decay is specifically the decaying of an unstable element/isotope. This radiation is different from light, where as light is electromagnetic radiation, radioactive decay emits alpha, beta, or gamma particles. These particles, (along with a high part of the spectrum of light), are what we call "ionizing radiation".
Your most common interaction with radioactive decay, is probably when ever you eat a banana. As you may know bananas are a good source of Potassium. Some of this Potassium, however, is Potassium 40, an unstable isotope of Potassium, with a half-life of 1.251 x 10^9 years (a pretty large amount of time relative your life). Because of this, you probably won't have to worry to much about eating bananas!
You should probably worry more about ionizing radiation in general. High frequencies of light can carry enough energy that they can affect the deoxyribonucleic acid (DNA) in your cells, sometimes causing that cell, and its "offspring" to become malignant cancer cells, which can lead to cases of Melanoma among other things.
Only light that has more energy than the visible light spectrum can do this though, so you don't need to worry about standing in-front of your microwave, radio, or putting your phone in your pocket, as the radiation emitted by these devices is completely harmless, in-fact, the lamp in your room is producing stronger radiation then these things. Just remember to wear your sun screen, and you should be fine!