What is 6th mass extinction?

Anthony Barnosky, the curator of the Museum of Paleontology at UC Berkeley said that species go extinct all the time. However, new species also evolve constantly. This means the biodiversity always remains constant. A mass extinction will take place when this balance is broken.[1] 
For the 6th mass extinction, Dr Richard Leakey, the world's most famous paleoanthropologist has explained that[2]
"It's the next annihilation of vast numbers of species. It is happening now, and we, the human race, are its cause. The statistics show that about 17,000 to 10,000 species die out every year from Earth which are at a rate comparable with an impact of a giant asteroid slamming into the planet, or even a shower of vast heavenly bodies."
Besides this, there are also many arguments on the six mass extinction event. Niles Eldredge mentioned one is to divide this event into two discrete phases[3]--

(1). The first phase started about 100,000 years ago when the first modern humans began to disperse to different parts of the world.                            

(2). The second phase started when humans turned to agriculture about 10,000 years ago.
This opinion is for we are in the middle of the extinction event as well. 

Reference:

1.http://www.livescience.com/13038-humans-causing-sixth-mass-extinction.html
2.http://www.mysterium.com/sixthextinction.html

3.http://www.actionbioscience.org/newfrontiers/eldredge2.html