Luminescence assays come in many different types. Basically luminescence assays are a chemical or enzymatic reaction that uses a substrate. When the substrate is acted upon by the chemical or enzymatic reaction, light is given off as a by-product. The two main types of luminescence reactions are Flash and Glow. The Flash luminescence reaction occurs quickly, in a matter of seconds or minutes, giving off a very bright signal. Whereas Glow luminescence assays can last for hours but they are not as bright as Flash luminescence assays.
An example of the Flash luminescence assay is DLR® from Promega: http://www.bmglabtech.com/application-notes/luminescence/dlr.cfm
An example of a Glow luminescence assays is the ADP-Glo® Kinase assay from Promega: http://www.bmglabtech.com/application-notes/luminescence/promega-adp-glo-kinase-assay-202.cfm