Pandora is free on the iPod touch, iPhone and iPad, however the free service is supported by advertisements and has restrictions. A paid Pandora subscription allows you to listen to higher quality ad-free music and gives your more "skips" than the free version. Subscriptions start at $3.99 a month.
You can download the Pandora Radio iOS app for free from the App Store. It requires iOS 5.0 or later.