Apple chose to limit direct printing from iOS devices to AirPrint enabled printers. That means one of the printers on the list must be connected to the network. In essence these printers send out a signal over the local network that your iPad can recieve via WiFi and work with with AirPrint.
There are work-arounds: You can install small application on your local network that will allow your iPad to recognize any printer on your network that has its preferences set to share. Any iOS device using the local wifi will immediately be able to send files to any printers recognized by the application. Search the OSX App Store. I found Fingerprint works for me. This setup allows you to print from any application using the AirPrint menu.
Another option is to use intermediate apps installed on your iPad that talk to network apps. An example is WePrint Server on Mac and PrintBureau on iPad - this is a more robust arrangement as it allows file transfers as well. Files within applications have to transfered to PrintBureau to print, so it is a two-step process.
Hope this helps.