Whether you enjoy laying in the sun on the beach, or hopping into a warm sunbed for a quick catnap the ratio of UV light is roughly the same. Both the natural sun and tanning units have a ratio of approximately 95% UVA and 5% UVB, with sunbeds being stronger to minimize the time needed to build a tan.
UVA and UVB are responsible for different aspects of the tanning process: UVB stimulates the production of melanin, but is also more intense and therefore more likely to cause a sunburn. UVA penetrates deeper into the skin and is the ray that actually darkens melanin in the skin. Both are needed to develop and maintain a tan.