Background & Introduction:
UV light has a short wavelength (~10 to 400 nm). UV has a lethal effect on most organisms primarily because of its ability to cause the formation of thymine dimers in DNA. Thymine dimers are two adjacent thymine bases that are abnormally linked together by covalent bonds. This dimerization inhibits DNA replication, which may lead to death of the organism.
Dimers form bumps in the DNA and disrupt the hydrogen bonding between bases on the complementary strands.
Since UV light exerts a lethal effect on bacteria it can be used to sterilize various objects such as benches, surfaces of foods, utensils, etc. Because UV light has very little penetrating power it is only used for sterilization of surfaces.
Because mutations such as thymine dimers are potentially life threatening, cells have a number of systems for finding and repairing damaged DNA. There are two primary mechanisms generally used for repair of UV damaged DNA. The first mechanism is called photoreactivation (light repair) and the second is generally termed excision repair. As excision repair takes place in the absence of light, it is also sometimes called dark reactivation.
Photoreactivation (light repair): The photoreactivation repair enzyme (PRE) is activated by visible light (400-750 nm). PRE uses blue light to break covalent bonds between thymine bases, allowing the hydrogen bonds to naturally reform.
Excision repair (dark reactivation): This repair process involves several enzymes:
a) An endonuclease called UvrABC1 breaks the sugar phosphate backbone of the DNA strand near the dimer on each side.
b) The H-bonds between the base pairs are broken and a segment of DNA ~12 nucleotides
long is excised.
c) DNA polymerase recognizes the 3’OH primer and fills in the missing bases. 
d) DNA ligase seals the final sugar-phosphate bond to repair the nicks.
1 More about this enzyme: Cell. 1983 May;33(1):249-60.
