The concept is simple. Amazon is trying to create artificial intelligence artificially. Most of the tasks can be performed by machines better than humans and then there are tasks that a machine can't even think of doing and humans are completely at ease with them. mturk uses image recognition as an example. CAPTCHA is designed around the premise that humans can read the images and machines can not. mturk calls these tasks HITs.
A Human Intelligence Task, or HIT, is a question that needs an answer. A HIT represents a single, self-contained task that you can perform to completion and collect a reward. You may do as many HITs as you like, whenever you like, in any order.
mturk helps everyone. People can get their work done at low costs from many people (use crowd-sourcing), people can make money and in turn Amazon can make money from exchange and then they must be hoping that some people end up spending money they earn on Amazon itself ... !!!!
Oh yes, the project is still in BETA and yet to catch up. When it is up and running, may be I can put my blog as a HIT ... ?