The Deccan is the southern peninsula of India, and traps or traprock is volcanic outflow, from the Swedish trappe "stairs", from its appearance. Some of the Deccan is covered by enormous traps, one of the largest episodes of volcanism in the earth's history. They were laid down over a geologically short period, mostly less than a million years, exactly at the time of the K-T extinction 65 million years ago, when the Cretaceous Period gave way to the Tertiary, and dinosaurs and so many other creatures became extinct.
The Deccan volcanism is therefore a counter-theory to the widespread view that an asteroid impact at what is now Chicxulub in Mexico did in for the dinosaurs. There is no doubt that an enormous impact did happen there at that time, but some doubt whether it would have been large enough to unleash the massive climate change over more than a year required to cause mass extinction. Whereas calculations of the output of gases (especially carbon dioxide) and debris from the Deccan suggest it could have had an even greater effect on global climate.
The modern basalt traps are 2 km thick and cover 500 000 km2, and could originally have covered three to five times this much. They arose from a hot spot or mantle plume of lava upwelling through the crust, which the island of India encountered as it moved away from Africa towards Asia. The hot spot is today the Piton de la Fournaise volcano on the island of Réunion.
Trapped helium 3 has been found in Deccan rocks 68.5 million years old. A high ratio of helium 3 to helium 4 indicates that the rock is reverting to the primordial conditions of the creation of the earth, that is the mantle is being melted and turned back into core material: this is also found in plume conditions today. So the mantle under the Deccan was being consumed volcanically at least 3.5 million years before it came out as the main eruptions (though there was some activity before the 65 million year mark.)
The Deccan Traps theory for the K-T extinction was first proposed in 1991 by Dewey McLean of the Virginia Polytechnic Institute.
There is evidence for other mass extinctions being linked with periods of flood volcanism, though there is also evidence for asteroid impact as well, in the case of the biggest, the Permian/Triassic extinction (associated with volcanic floods in Siberia): a candidate crater has been identified on the coast of Western Australia.
Dewey McLean: http://filebox.vt.edu/artsci/geology/mclean/Dinosaur_Volcano_Extinction/ - this is the best summary of the theory, but the author is very testy about his rival Luis Alvarez and the politics of the debate.
Helium isotopes: http://www.rochester.edu/pr/releases/ear/basu3.htm
Carbon dioxide: http://www.cnrs.fr/Cnrspresse/n396/html/en396a06.htm