Photocatalysis is a reaction which uses light to activate a substance which modifies the rate of a chemical reaction without being involved itself. And the photocatalyst is the substance which can modify the rate of chemical reaction using light irradiation.
In titanium dioxide the electrons or holes can diffuse to the surface and generate radicals. That leads to decomposition of organic substances. Especially the holes have a high oxidative effect. Out of water OH-radicals are formed and organic substances are decomposed by them. In many cases CO2 and water are end products.