Importance of sunscreen?

we are constantly informed about the importance of wearing sunscreen all the time--when we are tanning and when we are not. i don't understand exactly what sunscreen does. if people want to go out and tan, does sunscreen prevent a burn AND a tan, just prevent a burn, or prevent a burn and make the tanning process slower? i realize why it's important to prevent sunburns, but you only live once and a golden tan is really nice to have. i want to prevent skin cancer and burning while achieving that golden color. is that possible while wearing sunscreen? i am not naturally pale by the way, i have an olive tone.