the author of this app says:
GarageBand turns your iPad into a collection of Touch Instruments and a full-featured recording studio so you can make music anywhere you go. Use Multi-Touch gestures to play pianos, organs, guitars, drums, and basses on your iPad. They sound and play like their counterparts, but let you do things you could never do on a real instrument. Enjoy a full range of Smart Instruments that make you sound like a pro even if youve never played a note before. Plug an electric guitar into your iPad and play through classic amps and stompbox effects. Use the built-in microphone or a guitar to record, or capture any performance. Then mix up to eight tracks to create a song you can share.
Compatible with iPad. Requires iOS 4.2 or later.