Ultraviolet radiation arrives in packets of energy called photons. As each photon arrives on the skin it can cause damage. If the damage is done to the nucleus of a cell it can cause it to malfunction. As this cell then starts to reproduce it can develop into a cancerous tumour. This kind of tumour is called a melanoma.
For more information see here. In spite of this people will still sit in the sun, without wearing protective clothes or sunscreen.
What do you think are the possible reasons that people continue to sit in the Sun, even with the associated risks? Read more about people's attitude to risk here. In other parts of the world people are much more careful about their exposure to the Sun. How can people be persuaded to protect themselves?
The Ozone layer