To use the app, first play a song on your iPhone or iPod Touch. Then start the Scratch application and choose a sample. The free app has 10 samples to choose from, while a $2.99 Plus version allows you to record your own.
While you'll find a volume control and a sample timer you can use to move to a particular time in the sample playback, this is a just-for-fun app, and not a real DJ mixing program by any means. But it does offer some interactive fun with your music.
Note: The download link above will bring up the Scratch app page in iTunes on your PC. Click the 'Get App' button on the page and then sync your iPhone to install the app.