Extinction of a species ...

"Extinction of a species is always a concern as it is irreversible, and the loss of a species will mean the loss of unique combination of diversity at gene and organism level.  Because all species will eventually become extinct, those species at risk are said to be ‘threatened’."   http://www.beep.ac.uk/content/183.0.html

Extinction of a species may be sudden or extend over a longer period of time, often more than an average human lifespan. Accurate data for extinction rates are difficult but some are shown on this site ..... 

http://www.biodiv.org/gbo/chap-01/chap-01-02.asp

24% (1130) of mammal species and 12% (1183) of birds are globally threatened (figures for 2000)

The red list of threatened species is given at    http://www.iucnredlist.org/

Case Studies:

Passenger Pigeon :   Probably Once The Most Numerous Bird on Earth--  It Is Now Extinct

The Dodo :   http://www.birdlife.org/datazone/speciesfactsheet.php?id=2441

http://www.bagheera.com/inthewild/ext_dodobird.htm

By Pearson Scott Foresman [Public domain], via Wikimedia Commons

Mass Extinction Event :   http://paleobiology.si.edu/geotime/main/htmlversion/cretaceous4.html