The skin reactions by UV exposure are divided into

Acute reactions: come after a single, strong UV exposure quickly (sunburn)
Chronic reactions: often repeated by UV exposure over a long time (see below)
Abnormal inflammatory reactions:"sun allergy"
Reduced light tolerance by other causes - mostly by a gene defect caused (porphyries, Xeroderma Pigmentosum)
What skin damage UV radiation can cause long-term (chronic Light Reaction)? 
UV light, especially UVB, is mutagenic, which means it can be inherited (DNA) damage. UVB is directly absorbed by DNA, so it is stronger than UVA DNA damaging. In conjunction with UV radiation can be called oxygen free radicals arise. These are highly reactive substances that are cell components by oxidation damage or destroy them.
Most are UV-induced damage to DNA repair, defective parts are "cut out" and replaced by intact and it does not result in permanent damage.
When irreparable injury is a "sinking program activated and the cell dies from this (apoptosis). Were the damage caused by UV radiation triggered as a sunburn is the case, called such cells "Sunburn cells".