When your skin is exposed to UV radiation the pigment cells (called melanocytes) in the outer layer of the skin produce more pigment (melanin) which results in the skin darkening with a tan.
A tan can offer a small level of protection against sunburn (some estimate about the same protection as SPF 3 sunscreen) but a tan is a sign the skins DNA has been damaged by ultraviolet radiation, it is not a sign of good health.
Tanning without sunburning still causes damage and every time you expose your skin to ultraviolet radiation you will increase your risk of skin cancer.
The dominant ultraviolet ray that causes a tan is UVA.
In solarium/tanning salons, sun lamps are used to create a tan by irradiating the skin with UVA radiation 10-12 times stronger than from natural sunlight.
A solarium tan is not a safe tan and every exposure to UV radiation increase the risk of developing skin cancers. Studies have demonstrated rates of skin cancer are significantly higher in those who have used solariums. Use of solariums when young (in your teens) has been shown to increase the risk of melanoma by 75%.
A move to ban solariums began in 2009 when the World Health Organisation (WHO) classified tanning beds as a Class I carcinogen, putting them in the same category as cigarette smoking. Subsequently in Victoria bans on children under 18 years of age using solariums were introduced and now legislation has passed to totally ban all tanning devices as of the end 2014.
The take home message is there is no such thing as a healthy tan, a tan indicates injury to the skins DNA, and damage to DNA can lead to skin cancer and premature ageing of the skin.