If you don't have an iPhone or your not familiar with iTunes, you can find out all about it here: http://www.apple.com/itunes/whatis/
iTunes is a free application for Mac and PC. It plays all your digital music and video. It syncs content to your iPod, iPhone, and Apple TV. And it’s an entertainment superstore that stays open 24/7. You can buy applications like Shape Builder on iTunes or directly from your iPhone using the "App Store" program!