iOS AirPrint Printers http://support.apple.com/kb/HT4356
How to Print from Your iPad: Summary of Printer and Printing Options
Print from iPad / iPhone without AirPrint
How to Enable AirPrint on a Mac and Use Any Printer
iPad Power: How to Print
Check out these print apps for the iPad.
Print Utility for iPad ($3.99) http://itunes.apple.com/us/app/print-utility-for-ipad/id422858586?mt=8
Print Agent Pro for iPad ($5.99) http://itunes.apple.com/us/app/print-agent-pro-for-ipad/id421782942?mt=8 Print Agent Pro can print to many non-AirPrint and non-wireless printers on your network, even if they are only connected to a Mac or PC via USB.
FingerPrint turns any printer into an AirPrint printer
Go to this Apple support page:
There is a list of AirPrint enabled printers there and they are listed by manufacturer. If your printer is listed, turn it on. Then when in a document that can be printed select print and your printer will appear as a choice to print to.
Note that all Wi-Fi printers are not AirPrint capable.