If you take the time to read this blog last dozen posts, you'll see my struggles to understand and implement Google Voice.
It occcured to me I haven't really defined what GV is.
It is a telephone application from Google. It is not yet out to the public. I got lucky a year or so ago and got a invite to the Beta for GrandCentral that Google bought and transformed into GV.
It will be made available to the general public "soon" in Google parlance, which has already been almost two months. When it actually goes public is anyone's guess.
Lifehacker.com did a great profile of GV back in March. It is as good an introduction to GV as I can find.