system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What exactly is radiation? Why is it dangerous?

Also, if someone could explain why nuclear reactions give off so much radiation, that would be great.
gpt
Radiation is defined as waves or particles emitted from an atom as it moves from a higher energy state to a lower energy state. This comes in several forms: heat, light, beta (+/-), alpha, gamma, electron, proton. 

Radiation is dangerous because of the high energy with which the particles are emitted. These particles can penetrate the skin and enter the body, where they can damage cells and lead to cancer and other defects.