Firefly luciferase is a 61kDa protein isolated from beetles (Photinus pyralis), while Renilla luciferase is a 36kDa protein from sea pansy (Renilla reniformis).
These enzymes differ in their substrate and cofactor requirements. Firefly luciferase uses luciferin in the presence of oxygen, ATP and magnesium to produce light, while Renilla luciferase requires only coelenterazine and oxygen. Firefly luciferase produces a greenish yellow light in the 550–570nm range. Renilla luciferase produces a blue light of 480nm. These enzymes can be used in dual-reporter assays due to their differences in substrate requirements and light output.
More detailed information on luciferases can be found in the Bioluminescent Reporters chapter of the Promega Protocols & Applications Guide.