Cretaceous-Paleogene extinction event

Cretaceous-Paleogene extinction event, also called the K-T extinction where K stands for Cretaceous and T for Tertiary, is the most well known extinction event in the world and is the last one of the "Big five". It happened about 65 million years ago. During the this boundary of time, about 40-65% of marine species died out from the Earth ecosystem[1]. Almost all the large vertebrates on Earth, on land, in sea, and in the air suddenly became extinct including coccoliths, planktonic foraminifera, ammonites, gastropods, sponges, marine reptiles, dinosaurs and mammals. 
For this mass extinction event, there are still arguments existed about just how short it was[2]. It was certainly sudden in geological terms and may have been catastrophic by anyone's standards.

Why it happened? There have been many bad theories to explain this extinction event and the causes are still being debated by paleontologists. Based on researches, it is believed that a major factor was an asteroid about 10 kilometers in diameter that struck what is now the Yucatán peninsula in Mexico[3]. The impact led to many serious effects, for instance, global forest fires, possibly a period of cold weather because of sunlight-blocking dust and smoke, etc. Another hypothesis of the cause is a massive bout of volcanism.  Here is a video that gives an explanation to the K-T mass extinction in details.

Reference

1. MacLeod, N. in press. End-Cretaceous extinctions. In R. C. Selley, L. R. M. Cocks and I. R. Plimer (eds.), Encyclopedia of Geology. Academic Press, London.

2. http://www.ucmp.berkeley.edu/education/events/cowen1b.html
3. http://paleobiology.si.edu/geotime/main/htmlversion/cretaceous4.html