Solariums are chambers that contain artificial light sources that emit UV-A and UV-B radiation. They are designed so that people may lie in them and have the entire skin exposed for a period of time in order to induce a tan. Unfortunately this causes a significant amount of skin damage atthe same time and increases the risk of skin cancer by up to 75%. Some solariums emit more UV radiation in 20 minutes than the total dose of UV radiation that a person would receive at Bondi Beach during an entire day in the middle of summer.
There are other options that can give you a tan such as a fake-tan from a bottle, as they don't cause damage to the skin.