An extinction occurs when a population decreases to zero size. It is an inevitable event in infinite time given that zero is in effect an absorbing boundary. Thus, all populations must eventually go extinct; it is a natural process, as Jet-Poop has stated above. For natural populations of living organisms, extinction then becomes a question of when.
Current estimates of the rate of extinction indicate that in the next decade or so, the human population will have outdone a planetary asteroid strike as far as the proportion of species wiped out. Way to win one for the Gipper, folks!