PUVA is an acronym. The P stands for psoralen, the U for ultra, the V for violet, and the A for that portion of the solar spectrum between 320 and 400 nanometers in wavelength. Psoralens are chemicals found in certain plants that have the ability to absorb ultraviolet light in the UVA portion of the solar spectrum. Once the light energy is absorbed, these psoralens are energized to interact with DNA, ultimately inhibiting cell multiplication, which is its presumed mode of action.
Certain skin diseases are characterized by cells that are rapidly multiplying. Inhibiting this unrestrained proliferation can be useful in treating these diseases. So PUVA is a combination of an oral drug and subsequent ultraviolet light exposure.