The chief danger of radiation is the damage it can cause to living cells. Radiation can damage the DNA inside a cell's nucleus, and if the DNA becomes sufficiently damaged, the cell can become cancerous. Exposure to high amounts of radiation, or lower amounts over a long period of time, can significantly increase the risk of developing cancer.
There are a number of different types of radiation. Ultraviolet radiation is part of sunlight, and can cause skin cancer with enough exposure. Largely, however, the threat from ultraviolet radiation is greatest in areas of the globe where the ozone layer has become depleted. X-rays are also emitted by the sun but absorbed by the planet's atmosphere. Most x-ray exposure occurs as a result of medical exams.
Alpha, beta and gamma radiation are products of nuclear reactions, and all are dangerous to humans. Alpha radiation cannot penetrate the skin, but if a particle emitting alpha radiation is swallowed or breathed in, it can do significant damage to the cells in internal organs. Beta and gamma radiation have suitable wavelengths to penetrate the skin and damage cells inside the body. A radiation source may put out multiple forms of radiation, making any radioactive accidents extremely dangerous for those in the immediate area.