You need a printer capable of wifi connections... works pretty good with the one I have.
More than just WiFi. A printer with AirPrint functionality is preferable. This feature will be prominently mentioned on packaging and in advertisements.
For example, I bought this one for $40 on sale a few weeks ago. Regular $80.
HP Wireless All-In-One Inkjet Printer (3050A)
"AirPrint technology allows you to print wirelessly from your iPad, iPhone or iPod touch"
The above printer also works well with the VueScan app ($5) for scans to iPad or iPhone. Only good for low volume printing.
-You *can* use "ePrint", found on some printers as well:
"ePrint mobile printing - if you can e-mail it, you can print it"
ePrint is considerably less pleasant to use vs AirPrint. You need to email attachments to the printer instead of just pressing print. I also had a 6MB PDF doc (30 pages) which wouldn't ePrint. "Attachment too large".