For the fake bake detesters out there, gradually building up a tan from a tanning bed is much less harmful than going on vacation to a tropical island and exposing your pale, virgin flesh to the intense tropical sunlight.
Tanning itself isn't that harmful. Burning your skin from tans is bad. Also, if you naturally have a higher amount of melanin in your skin you will not burn as easily as someone who has paler skin. The less easily you burn, the lower chances you have of skin cancer.