Shorter URLs are special services that allow you to shorten web page addresses so you can easily insert them into posts where there is a limit of characters (example flashy Twitter).
There are hundreds of sites that allow this (same as Google offers this opportunity), but around the web there are also portals that guarantee earnings in using their service.
By clicking on a shortened link from these particular sites that promise remuneration, the user you visit will be forced to stay a few seconds on an ad page (each sponsored view, the person who shortened the URL will receive a fee).
The gains are not very high and are based on the number of views a user can receive (this means that only those with big traffic can actually earn).
That said, for us in GetMyRefs is still an opportunity to exploit and that could come back very comfortable to the most.
Our advice is, however, not to abuse too much in inserting short URLs. As we have reported in the "cons", one of the worst aspects of short URLs is the boredom of the service: most users do not like waiting for 5 seconds to go to a page of their own interest (often close it and go away). In this regard, we recommend centrelining the use of these services and using them once every 7/8 post on average.