It runs in the background and basically allows you to print from your iOS device (I have an iPhone) to your printer. The printer doesn't have to be wi-fi enabled or anything, as the process works through the computer.
It will even print to a PDF (eg, Adobe Reader will open on the screen with the page).
When I bought it, it was US$9.99, but seems to have gone up to $19.95.
Works great for me. When I choose "Print" in an iOS app, my printer shows as available to print to.
Disclaimer: I am not involved with the maker of the app in any way, I just thought it might be useful for people to know about.