Here's the link
https://www.mturk.com/mturk/welcome
Now the tasks that they want you to do are varied. They are called HIT's [Human Intervention Tasks]. Basically these are jobs that are more or less mundane and boring stuff.
Some examples of what I did which got me some moolah [Which I can't anyway encash, because I haven't hit the required 100$ mark]
1. Posting information about a product by looking at the picture
2. Writing comments on blog
3. Writing product reviews
4. Writing movie reviews for video libraries
5. GIS Image interpretation [Mind-Numbingly boring] Requires you to draw boundaries for the street objects using the photos given by the application. This is how all the GIS apps map physical data into graphical maps. You can find a lot of HIT's then. I don't see much now.
And it goes on and on, you can register and do few if you like. But just like exercising, you seriously need commitment to make money out of this. They pay in denominations of 1cent, 5 cents, like that. Very rarely you will see someone pay more than a dollar. And the task would be much more extensive.
One more thing, the money is not immediately credited to your work. They evaluate and post passing evaluation only they credit the money.
Generally you have to be really lousy in what you did to lose money on a HIT. Most of the times the money comes in.
Hope this helps.