Final answer:
Gamma rays are a form of high-energy electromagnetic radiation that possess tremendous penetration power and have the least ionizing power. They are not particles and can pass through dense materials without striking anything.
Step-by-step explanation:
Gamma rays are a form of high-energy electromagnetic radiation, similar to X-rays but more powerful. They have no mass or charge and are not particles. Gamma rays possess tremendous penetration power and can pass through several inches of dense material like lead without striking anything. They have the least ionizing power but the greatest penetration power. Gamma rays are often emitted by a nucleus during a transition between nuclear energy levels, and they have much larger energy differences compared to electromagnetic radiation produced by electronic transitions.