The sun and tanning beds both emit the same sort of ultraviolet (UV) radiation, comprised of about 95% UV-A radiation and about 5% UV-B radiation.
The difference between UV from the sun and UV from tanning beds lies in the intensity of the radiation. UV radiation from a modern tanning bed is 3-5 times more intense than UV from the sun. But this increase in UV intensity does not make tanning bed UV more dangerous than solar UV because of the limiting factor of time spent in the tanning bed.
The time spent in a tanning bed is limited to offset the greater intensity of the UV radiation. Indoor tanning sessions are typically limited to 75% or less of the time that it would take to get a sunburn from the tanning bed.
As the American Cancer Society acknowledges:
Suntan or Booth Tan: Your Skin Can’t Tell the Difference