Ultraviolet radiation is a type of electromagnetic radiation with wavelengths shorter than visible light but longer than x-rays. There are three main types of ultraviolet radiation - UVA, UVB, and UVC. UVA and UVB can affect humans and cause sunburn, damage skin and DNA, increase risk of skin cancer, and weaken the immune system over time. While some ultraviolet radiation is useful in small amounts for treatments, exposure is generally harmful and linked to premature skin aging and cancer development.