It seems like certain links from SE sites to certain other sites get implicitly converted -- or used to be -- to links of the format
An example is this answer to Where to get Best resources on D language?. It links to http://rads.stackoverflow.com/amzn/click/0321635361, which, after making SE a hundred-thousandth of a cent, redirects me to http://www.amazon.com/dp/0321635361/?tag=stackoverfl08-20.
What are these, and where can I find more information about them? Are the posters making a profit by using them?